New PGen infranstructure

[haykh]

• removed redundant srpic/ grpic/ prefixes from pgens
• support for external pgens
• add a separate pgen-repository as a submodule

can now compile with any external problem generator defined in arbitrary location. e.g., create mypgen/pgen.hpp anywhere. then simply pass via absolute:

cmake -B build -D pgen=<PATH_TO>/mypgen

... or relative path:

cmake -B build -D pgen=../mypgen

Also you can use the problem generators from the separate entity-pgen repository:

cmake -B build -D pgen=pgens/<PGEN>