solver backends should support multiple compute options

[kecnry]

see #877 for a use case and example:

import phoebe

b = phoebe.default_binary()
b.add_spot(component='primary', relteff=0.8, feature='spot01')

# add multiple datasets, each dataset will either have the spot enabled or disabled via separate compute options
# for this example, we'll just add one of each
b.add_dataset('lc', compute_phases=phoebe.linspace(0,1,101), dataset='lc_with_spot')
b.add_dataset('lc', compute_phases=phoebe.linspace(0,1,101), dataset='lc_without_spot')

# now we'll create separate compute options: one which will handle all datasets with the spot enabled and one
# for all datasets with the spot disabled
b.add_compute('phoebe', compute='comp_with_spot')
b.set_value('enabled', dataset='lc_without_spot', compute='comp_with_spot', value=False)

b.add_compute('phoebe', compute='comp_without_spot')
b.set_value('enabled', dataset='lc_with_spot', compute='comp_without_spot', value=False)
b.set_value('enabled', feature='spot01', compute='comp_without_spot', value=False)

b.run_compute(compute=['comp_with_spot', 'comp_without_spot'])

b.add_solver('optimizer.nedler_mead')
# want to be able to support something like the following by using a Select instead of a Choice parameter:
b.set_value('compute', context='solver', value='comp_*_spot')