Cling should never see this; add comment about libCore/libCling use; constness; default ctor = default.

This file is needed by libCling (dictgen) *and* libCore (TClass/TSchemaRule).
Luckily it is inline-only, so we simply #include it from both.

Basic, ROOT-independent interfaces (stringview, TClassEdit) go to core/foundation.
metautils becomes clingutils; hide TClingUtils (former TMetaUtils) from include/.
TCling* move to core/metacling/src/.
I/O related code for rootcling moves to core/metacling/.
Other dictionary-generation related code moves to core/dictgen.

- in group TMVA
- remove trailing spaces
- spell check
- math formula

- in group TMVA
- remove trailing spaces
- spell check

- in group TMVA
- remove trailing spaces
- spell check

- in group TMVA
- remove trailing spaces
- spell check

- Doxygen format
- in group TMVA
- remove trailing spaces
- spell check

- Doxygen format
- in group TMVA
- remove trailing spaces
- spell check

- Doxygen format
- in group TMVA
- remove trailing spaces
- spell check

*.icc is used to define the templated part of some classes. These files only
make sense if they are included by the header whoose templates it defines.

In the context of ROOT at cmake configure time, cmake looks tries to compile
code (testing for available compiler features) using -fmodules and the ROOT
modulemap. It errors out saying the $ROOTSYS/include/GL folder doesn't exist
because move_headers target is not yet executed.