Removed all uses of I as a variable to fix conflict with complex.h

[JeffreyBroll]

Address #62 by removing uses of `I'

[JeffreyBroll]

I think I got them all.

[drsteve]

Working through this, I was concerned that changing the structure members from I to something else (e.g., I_data) might break dependent code.
While we can't trap everything that users may have wanted, we can ensure that we don't break our own tools or examples. I haven't checked the Examples directory, but Tools/MagEphemFromSpiceKernel.c and Tools/MagEphemFromTLE.c are definitely affected. It's likely that the same is true for the MagEphem examples. The Python interface should also be checked... (and it'd be nice to just triple check that the name really needs changing in the structure to avoid the clash).

I'll also need to update some local code that uses I from the structures, so we'll have to make sure this (fairly high chance of breaking things) change is recorded in the changelog to let other users know.

[JeffreyBroll]

I was considering, as an extreme(ly silly) precaution, that we could raise their macros with another macro to re-rename new things back to old... but really I'm just surprised that this collision hasn't become an issue already. I'll start updating and checking examples.