use uuid instead of 'is' for selecting ensembles in rate calculation - #919

This came up on experimental storage, but should also be a bug on master.

If you load your network from one Storage object, but your steps from another (loading the same file), you might end up with 2 different instances of the same Ensemble.
The is evaluation is then too strict to handle this.

I opted to convert this to an uuid == uuid

I am having trouble setting up a test case for this, so if you have any pointers for that @dwhswenson, please let me know.

I would envision something like this should fail on master (but I don't have any workable .nc files at the moment, and couldn't quickly spot one in the test directory)

import openpathsampling as paths

s1 = paths.Storage("test.nc", 'r')
s2 = paths.Storage("test.nc", 'r')

network = s1.networks[0]

network.hist_args['max_lambda'] = { 'bin_width' : 0.005, 'bin_range' : (-3, 3) }
network.hist_args['pathlength'] = { 'bin_width' : 5, 'bin_range' : (0, 50000) }

assert network.rate_matrix(s1.steps, force=True) == network.rate_matrix(s2.steps, force=True)