Reinstated API:
-new spectrum command (specum_pi/ss now legacy)
-added warnings for legacy
-all correlation functions now call _correlation_2t
-added new test functions for new functions over legacy functions
-refined doc strings

todo:
-add mcsolve 'steady state'
-suppress mcsolve output
-something is broken about the test for the mc correlator - need to suppress mcsolve output so I can see what this is...