Add function for getting basis vectors from spice

[subagonsouth]

Change Summary

Overview

Adds a function for getting basis vectors (for example spacecraft axes) of one frame expressed in another frame.

Updated Files

• imap_processing/spice/geometry.py
  • Fixes a small bug in where the @ensure_spice decorator was used. It should be used at the lowest possible level, closest to the spiceypy function.
  • Adds function for getting basis vectors
• imap_processing/tests/spice/test_geometry.py
  • Adds test coverage for basis vector function

Closes: #794