Cassini ALE driver. (#110)

* Add getPosition with coefficients

* Add some exceptions and associated test

* remove debug statements and change test name

* Updated to use gsl for polynomial evaluation and separated out into a helper function. Swapped order of coeffs, other changes in response to review comments

* Added a couple of suggested tests and updated documentation

* Inital work toward ALE cassini driver

* Updated comments in base.py and removed notebook from this PR

* Update mk location to be accessible beyond my computer

* remove newlines

* Update name of Cassini ISS Ale driver

* Update comment based on feedback