emcee move feature

[kecnry]

This implements the ability to override the move within the emcee sampler backend via a feature but has not been tested.

To use:

import phoebe

b = phoebe.default_binary()
...
b.add_solver('emcee', solver='emcee01')
b.add_feature('emcee_move', solver='emcee01', feature='move01')
print(b.filter(feature='move01'))

TODO:

• test to see if this works and is actually useful to have in the code
• documentation
• test coverage