Improve O2 linter

[vkucera]

external-pi

• Fix false positives where M_PI is preceded by a character.

pi-multiple-fraction

• Fix false positives for multiples of correct constants for pi fractions (e.g. 2 * PIThird).

pdg/explicit-code

• Improve matching pattern to spot more cases, especially comparison expressions.

name/function-variable

• Fix false positives for concepts and explicit constructors.

name/constexpr-constant

• Test only part before the first # to avoid false positives inside macros.

name/o2-column

• Test only parts before the first # to avoid false positives inside macros.

name/o2-table

• Test only part before the first # to avoid false positives inside macros.

name/type

• Detect also concepts to be treated as types.

name/workflow-file

• Detect from the path, whether to expect a mandatory PWG prefix (currently only for HF) in the struct name or an optional one (other PWGs).
• Look for any struct name that is an identifier.

Extend the message about disabling to clarify that the option exists for exceptional cases and require a comment with a reason for disabling. This will help to see whether a test was disabled because of a false positive, special case, misunderstanding or is just hiding bad code.

Report numbers of disabled issues.