Update ISIS Label mix-in for refactor (#140)

* 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

* Modify IsisLabel driver for refactor. Update test

* Create new test for ISIS Label mixin, remove changes to ISIS spice test, and updated Isis Label for review comments