Fix initial theta grid size potentially unset in call to eikcoefs

Fixes #111

For Miller/local and EFIT equilibria only, it was necessary to call geometry::init_theta before the call to eikcoefs, while for the remaining numerical equilibria, a call in eikcoefs to tdef was necessary after reading the equilibrium file.

Additionally, tdef and init_theta were essentially identical, with the exception that tdef forced ntheta to be even.

This commit will:

• Replace tdef and init_theta with init_uniform_theta_grid

• Always call init_uniform_theta_grid inside eikcoefs in the appropriate places

• Make init_uniform_theta_grid private; it is now always called appropriately, so no other calls are needed

• Replace duplicated calls to tdef with single call to init_uniform_theta_grid

• Make the ntheta_returned out-parameter to ekicoefs no longer optional, and make it intent(inout); callers now always get the updated value

• Add docstrings to various touched variables/procedures, making sure they are all consistent