always use standard C++ regex

[ramcdougal]

Looks like there are two custom regex (or partial regex) implementations: src/oc/regexp.cpp and src/ivos/regexp.cpp.

std::regex has been part of the C++ standard for over a decade. Can we use that?

Remove:

• src/ivos/regexp.cpp Remove usage of IVOS/regexp #2204
• src/oc/regexp.cpp Replace hoc_regexp_* by std::regex #2694

[pramodk]

Linking # #2204. cc'ing @alkino !

[alkino]

ivos/regex.cpp use an unsupported option std::multiline. But I check all usages in nrn_modeldb_ci, everything is simple and does not need that.

Maybe we can merge it :$. I will look for the other regex.

[alkino]

Blocked by: BlueBrain/neurodamus#78