FiniteStrainMultiPlasticity refactored and it looks much nicer in parts now.

However, it's still huge, so this addresses #3879 but does not fix it entirely yet

Also, changed no_trap_fpe to no-trap-fpe in the test suite.  That possibly fixes #3882.
Merge pull request #3896 from WilkAndy/multi_refactor_3879

FiniteStrainMultiPlasticity refactored and it looks much nicer in parts ...