Implementation of the drift_bounce_orbit python wrapper

[AntoineBrunet]

This PR follows the issue #47 about having a python wrapper for the drift_bounce_orbit routine. It's a straightforward adaptation of the drift_shell wrapper.

I fixed a small issue in the drift_shell wrapper for the data type of the nposit variable. The documentation mentions a long integer datatype, but that is just IDL lingo for a 4-byte integer (which is simply called an integer everywhere else).

[PluckZK]

Thank you very much!

[mshumko]

Hi @AntoineBrunet, thanks for wrapping that function! May I suggest an edit to the docs and an addition of a test?

The function has alpha and R0 kwargs that are not included in the docstring. Would you mind adding this in?

R0: float
            The radius, in units of RE, of the reference surface (i.e. altitude) between which 
            the line is traced.
alpha: float
            The local pitch angle.

Also, can you add a function to test_IRBEM.py to automatically validate a subset of the outputs?

[AntoineBrunet]

Hi @mshumko,
Good remarks. It's always hard to test this type of output, I've tried to add tests that have a physical sense. Hope that's ok to you.

[mshumko]

Thanks for adding tests @AntoineBrunet!