* added a paragraph on "creating a TDataFrame", including a TDF from scratch
* added examples and short explanations for string expressions for Define and Filter
* added a minimal example of creation of a TDF from scratch, including snapshotting to a new data-set
* switched "branch" with "column" where we are not specifically talking about a TTree

Inside the ns are:
* TCustomColumn[Base] (was TDataFrameBranch[Base])
* TRange (was TDataFrameRange)
* TFilter (was TDataFrameFilter)
* TDataFrameImpl
* BranchNames_t (was in namespace ROOT)

An empty-source dataframe constructed with a number of entries will
generate those entries on the fly when some action is triggered,
and it will do so for all the previously-defined temporary branches.

this has no effect on the interfaces but allows to cleanly return a new
TDataFrame from the TDataFrameInterface.

Markdown tables seem to not like line breaks inside a cell.
Clang-format, on the other hand, did not like our very long lines,
breaking the docs.

for the case where the user specifies the new quantity as a string
containing a C++ expression.

indeed there may be situations where two workers start processing
the same slot: there was no guarantee that the slot number which
was delivered by the circular buffer had not being assigned to
a worker which did not finish yet its work.

the slot number acquisition mechanism was based on a map which had
as key the threadID and as value the slot number. This is not
necessary because every operation is identical irrespective of the
slot - in some sense it is thread-agnostic. A trivial lock-free,
read-only buffer was implemented as helper class of the
TDataFrameImpl::Run method. In addition, the new method allows the
runtime to replace existing threads with new ones: no new entries
in the map are necessary and the total number of slots assigned to
the Operations can stay a constant decided before looping.

- add Histo{2D,3D}, Profile{1D,2D} to docs
- Extra actions -> Queries
- temporary branch -> temporary column
- Histo -> Histo1D

instead of pre-compiling the calling sequences for a few types,
jit the adequate CreateAction call after reconstructing the name
of the type from the TBranch or the volatile added column.