add stepping stone method for calculating evidences

[meyers-academic]

Add stepping stone method for calculating evidences. Uses la_forge.slices.SliceCore object (which is still only on dev branch). Include "moving block bootstrap" for calculating errors on evidences.

Example usage is fairly simple:

from la_forge import slices, bfacs
m3a_slice = slices.SlicesCore(label='pat', slicedirs=['12p5yr_ti/12p5yr_ti_3a/'], pt_chains=True, pars2pull=['lnlike'])
evidence, evidence_error = bfacs.calc_stepping_stone(m3a_slice, bs_iterations=100)

Still to finish:

• Unit test comparing method to analytic evidence calculation
• Proper documentation and warnings about MBB method for getting error (may need to maximize over block)