EPL Documentation / Edge cases

[smericks]

• remove stellar_mass as a required parameter in both epl and epl_sersic docstrings
• change angular_size to an optional parameter in the epl docstring (see test_epl.py, angular_size is never used)
• add center_x, center_y as optional parameters in both epl and epl_sersic docstrings
• currently, a user is allowed to provide both a v_disp and a theta_E. But, there is no guarantee that the two quantities are consistent, especially if the cosmology changes. I introduce a check for this edge case, and raise an error if both are provided. There is no way to check if the two quantities are consistent without assuming a cosmology model.
• I also check if a user tries to compute theta_E without providing a lens_cosmo, raising a useful error message (not sure the convention for this)

[smericks]

Seems like not allowing a user to provide both v_disp and a theta_E breaks an existing test cases in tests/test_Lenses/test_lens.py, because at line 537, both a v_disp and theta_E are provided when making a mock lens for a test case.

AND note to self for next commit: angular_size is in arcseconds, not radians... (see theta_E_from_vel_disp_epl())