Stylistic changes proposed by bjornbm.
Implemented combination of models with different reference radii.
Remove unneeded export.
Added VectorSpace instance for spherical harmonic models.
Cleaner implementation of changing reference radius without numerically inverting the triangle function.
Removed export of helper function.
Naive attempt to provide gradient in cartesian coordinates.
Stylistic simplification of model evaluation.
Changed evaluation function to work with triangular list of coefficients.
Simplified data type by eliminating redundant fields.
Added a more direct implementation of Cartesian evaluation that does not round trip through (inverse) trig functions.
Simplified addition of spherical harmonic models.
Reordered declarations. Added documentation. Cleaned exports.
Added documentation.
Began development on 0.3. Merged changes addressing #4 and partially addressing #5.