Consistent ffi

[jhod0]

This PR makes two main changes for consistent interfacing with C code, and error handling:

• Passing arrays to C routines is now handled with the _ArrayWrapper class, which always ensures arrays are the right type, and contiguous in memory (to avoid memory corruption). It also consistently handles passing single values to C routines.
• C routines using GSL functions now return the GSL error code to the Python wrapper. If retcode != GSL_SUCCESS, an error is thrown in Python. This ensures that errors are always detected, and makes it possible to handle them explicitly in Python.

[jhod0]

Seems like the first bullet above addresses #9 as well