p_from_z and z_from_p: add two arguments

[efiring]

The two arguments, geo_strf_dyn_height and sea_surface_geopotential,
are not used internally and are optional in the Matlab and Fortran
versions. They will also be optional in the Python version. C support
for optional arguments is less straightforward than in the other
languages, so we stick with the simple 4-argument signature instead.

[DocOtak]

Oh my... look at all that trailing whitespace that is all my fault.

This PR looks ok to me, but I probably won't have an opportunity to test myself (currently on the TGT).

[efiring]

@DocOtak Thanks for taking a look from your very remote location!
I ran the standard test, which I modified to include the new args set to zero. Presently there are no tests for the optional args, either in C or in Matlab; I can make a test for that in the Python version.