add util to build tripolar grid

[raphaeldussin]

PR #223 uses data files that are too big, this PR adds code to build a simple (simplistic?) tripolar grid for testing purposes only. PR #223 could then use this function to generate its own tests with synthetic data from a tripolar grid to a regular grid using some type of field (e.g. sin(lat) )

[raphaeldussin]

@huard @aulemahal this implements the simple_tripolar_grid function whose purpose you've already figured out. The remaining routines are copied from a GFDL package rather than adding a new dependency for very little code. This will help solve issues regarding testing with data from tripolar grids.

[huard]

I suggest making bipolar_projection and generate_bipolar_cap_mesh private functions.

[raphaeldussin]

@aulemahal friendly reminder

[aulemahal]

I do not have much knowledge of tripolar grids, but this code looks fine!