Improve use of ROOT dictionary generation macros

Summary:

• Remove all uses of deprecated ROOT ClassImp macro
  • As noted in T213, this prevents compilation in Belle 2 software stack due to a deprecation warning being promoted to an error by -Werror

• Generate ROOT dictionary only for classes that need it for I/O
  • As noted in T213, dictionaries are only required for classes that need I/O
    • In our case that is only, currently, LauAbsRValue, LauBlind and LauParameter
  • Remove all but these from the LinkDef file and the dictionary generation command
  • Remove ClassDef macros from all other classes

• Take advantage of related improvements in the ROOT CMake support
  • Consequently bump the minimum ROOT version to 6.18.

Test Plan:
Examples, particularly those running a simultaneous fit (which uses the I/O of
LauParameter to TBufferFile), continue to run ok

Reviewers: jback

Reviewed By: jback

Tags: Laura

Differential Revision: https://phab.hepforge.org/D101