ENH: vector transformation

[aburrell]

Is your feature request related to a problem? Please describe.
@jcoxon informed me of some work Colin Waters had done to allow AACGM to do vector transformations between geodetic and magnetic coordinates.

Describe the solution you'd like
Add two functions for vector transformations: one that works on a single value, and another that works on an array of values.

Describe alternatives you've considered
Work with Simon to put the code in the C-code base.

Additional context
Seems easier to add this as a special option to the python package.