User-defined features

[kecnry]

This PR adds support for defining custom features in-line (and likely supersedes #891 and eventually #892).

Current "simple" example:

import phoebe

b = phoebe.default_binary()
b.add_dataset('lc', compute_phases=phoebe.linspace(0,1,101))

from phoebe.features import DatasetFeature
from phoebe.parameters import FloatParameter, ParameterSet, constraint
from astropy import units as u
import numpy as np

class SinusoidalThirdLight(DatasetFeature):
    #allowed_dataset_kinds = ['lc']

    @classmethod
    def get_parameters(self, feature, **kwargs):
        params = []
        params += [FloatParameter(qualifier='amplitude',
                                  latexfmt=r'A_\mathrm{{ {feature} }}',
                                  value=kwargs.get('amplitude', 1.0),
                                  default_unit=u.W/u.m**2,
                                  description='Amplitude of the third light sinusoidal contribution')]
        params += [FloatParameter(qualifier='period',
                                  latexfmt=r'P_\mathrm{{ {feature} }}',
                                  value=kwargs.get('period', 1.0),
                                  default_unit=u.d,
                                  description='Period of the third light sinusoidal contribution')]
        params += [FloatParameter(qualifier='freq',
                                  latexfmt=r'f_\mathrm{{ {feature} }}',
                                  value=kwargs.get('freq', 2*np.pi/3.0),
                                  default_unit=u.rad/u.d, advanced=True,
                                  description='Frequency of the third light sinusoidal contribution')]

        constraints = [(constraint.freq, feature, 'feature')]

        return ParameterSet(params), constraints

    def modify_model(self, b, feature_ps, model_ps):
        from astropy import units as u
        import numpy as np
        _skip_filter_checks = {'check_default': False, 'check_visible': False}

        period = feature_ps.get_value(qualifier='period', unit=u.d, **_skip_filter_checks)
        t0 = b.get_value(qualifier='t0', context='system', unit=u.d, **_skip_filter_checks)
        for flux_param in model_ps.filter(qualifier='fluxes', **_skip_filter_checks).tolist():
            ampl = feature_ps.get_value(qualifier='amplitude', unit=flux_param.default_unit, **_skip_filter_checks)
            times = model_ps.get_value(qualifier='times', dataset=flux_param.dataset, unit=u.d, **_skip_filter_checks)
            flux_param.set_value(flux_param.get_value() + ampl * np.sin(2 * np.pi * (times - t0)) / period, ignore_readonly=True, **_skip_filter_checks)

b.add_feature(SinusoidalThirdLight, dataset='lc01')
b.run_compute()

TODO:

• support for component features (and example)
• try to remove need to define imports in-method (or document otherwise)
• support overwriting code for an existing feature (perhaps just with add_feature(..., overwrite=True, **b.get_feature(...))
• decide between registering vs storing code, etc.
• entries need to show up in list_available_features
• example for migrating spots
• example for sinusoidal intensities
• test coverage
• documentation