Sinusoidal Intensities feature

[kecnry]

this PR implements:

• the ability for (component) features to hook into and modify mesh intensities
• a new feature that adds a sinusoidal perturbation term to the mesh intensities before eclipsing/integration

import phoebe

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

b.add_feature('sinusoidal_intensities', component='primary', feature='puls01')
b.set_value('period@puls01', 0.25)
b.set_value('amplitude', 0.2)

b.run_compute(model='with_puls')
b.plot(save='sinusoidal_intensities.png')

TODO:

• fix issue with reloading bundle after saving with feature added (requiv filter somewhere needs to set the context)
• test coverage
• t0@feature parameter with default constraint to t0@system
• documentation
• test coverage
• consider as time dependent unless same period (or fraction) as orbital period AND parent star is synchronous?