...and add a test.

The new argument is to make TTreeProcessorMT consistent with other
multi-threading interfaces, namely TThreadExecutor. This is the last
ingredient required to fix ROOT-10561.

...to make it clear that we only use these functions in this same
source file.

Co-Authored-By: Stephan Hageboeck <stephan.hageboeck@cern.ch>

Internal logic has been adapted to support the case in which TTreeProcessorMT
is acting on a TChain and the trees in the TChain do not all have the same name.
Importantly, the fTreeName data-member changed name to fTreeNames and is now a
vector of strings rather than a single string.

This fixes ROOT-10178.

This is to fix ROOT-10549.

Before, the TThreadedObject had to guess 64 slots were enough for the use case of TTreeProcessorMT - and with 256 cores that was not the case.
Be explicit about the use of IMT for TThreadedObject by providing the new argument.
IMT is enabled before the construction of the TThreadedObject (and thus TThreadedObject now knows how many slots to expect), because the pool is constructed before.
If the order ever gets changes, the TThreadedObject will complain as it is now told to expect IMT usage.

A future TTreeProcessorMT ctor overload might provide a specific thread count, and can pass it down to TThreadedObject.

Move the creation of `TThreadExecutor pool` before the call of `Internal::MakeClusters()`, to make sure it is properly initialized when calling `ROOT::Internal::TPoolManager::GetPoolSize()` (via `ROOT::GetImplicitMTPoolSize()`)
This fix a crash with the `gtest-tree-treeplayer-test-treeprocessormt` test on Windows, due to a division by zero at line 224 in `TTreeProcessorMT`

* Retrieves correctly the name of the trees building the chain if the
  chain itself does not has a name. This happens if the default
  constructor for TChain is used and the name of the tree is retrieved
  via the string given to `TChain::AddFile("file.root/treeName")`.
* This does not fix the issue entirely since the FriendInfo needs a
  refactoring. The FriendInfo assumes that in a chain each tree has the
  same name, which could not hold true.

Remove the typo that caused "Last cluster in processed TTree is
dropped under specific circumstances".

for example when the TEntryList contains sub lists, matching with the
name of the file and tree whenever needed.

the issue is that, under certain circumstances to be clarified, the TEntryList::GetEntry
method returns -1 even when it should when called for the first time.

this is achieved adding a special case which checks if the directory
containing the tree is a file or a proper directory.

and simplify the implementation of the algorithm which reduces the number of ranges.

and not one task per cluster.
We need to "fuse" together clusters if the number of clusters is to big with respect to
the number of workers, otherwise we can incur in an overhead which is so big to make
the parallelisation detrimental for performance.
For example, this is the case when following a merging of many small files a file
contains a tree with many entries and with clusters of just a few entries.
The criterium according to which we fuse clusters together is to have at most
TTreeProcessorMT::GetMaxTasksPerFilePerWorker() clusters per file per slot.
For example: given 2 files and 16 workers, at most 16 * 2 * TTreeProcessorMT::GetMaxTasksPerFilePerWorker()
clusters will be created, at most 16 * TTreeProcessorMT::GetMaxTasksPerFilePerWorker() per file.

in addition, remove the MakeClusters helper function from the header file.

also for chains initialised as
```
TChain c;
c.Add("my/path/to/file.root/tdirectory1/.../tdirectoryN/treename")
```

excluding it from the processing.

This should fix clang's compilation error:
"constructor for 'ROOT::TTreeProcessorMT' must explicitly initialize the const member 'fFriendInfo'"

Previously all the dataset information was stored in the thread-local
TTreeViews, causing data duplication between threads and requiring
that TTreeProcessorMT queries it when it needs part of it.
Now TTreeProcessorMT stores all the dataset information (tree name,
filenames, friend names, friend file names, entry list) as constant
thread-global data which is passed down to TTreeView::GetTreeReader.