Remove pointless macros from `crout.hpp`

[1uc]

The naming conventions in solveCrout need fixing. y isn't a good name for a pivot vector, p is more canonical, but p is something completely different, unless it's used through the macro y_[i] (which is p[y[i]]).

The the blocks of code are completely duplicated and can be trivially be deduplicated using a sensible abstraction for y_.

The second copy of the comments is incorrect/misleading about y.