This reverts part of commit 5059f594.

Prefer string_view in function signatures. Avoid instantiating
strings whenever possible.

Also add accessor with protected visibility for the one that needs it.

(also avoid an extra shared_ptr copy)

* 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.