Only document the long-form arguments

[seisman]

Description of the problem

GMT command line and its C API only accept single-character arguments, e.g., -B and -J, whereas Python and PyGMT users mostly would prefer long-form arguments instead, e.g., frame for -B, projection for -J. Currently, PyGMT supports both single-character and long-form arguments, but we definitely will disallow single-character arguments in the future. See issue #262 for the reasons.

Currently, the documentation is pretty confusing. Take the documentation of Figure.coast() as an example. Below is what it looks like now:

For PyGMT users who never used GMT before, it's not clear what the aliases mean. I believe most of them are confused about the inconsistency of the single-character arguments in the coast documentation and the long-form arguments in the tutorials and gallery.

It will be a lot of work to fully implement #262, but at least now we should stop documenting single-character arguments and document long-form arguments instead.

What we can do:

• Replace all single-character arguments with the corresponding long-form arguments, e.g., B->frame, J->projection.
• Remove the aliases from the docstring

[weiji14]

A quick way to catch most of these is by fixing the common aliases (e.g. R=region, J=projection) listed in https://github.com/GenericMappingTools/pygmt/blob/master/pygmt/helpers/decorators.py. Should add a few others as well (e.g. l=label for legend, and c=axes for subplot once #412 is sorted). We can then slowly handle all of the other edge cases.

Edit: I've removed most of the short-aliases (except R, J, B, CPT, G, W, U) for plotin the plot3d PR (since they were basically the same), will be visible once #471 is merged in, preview at https://pygmt-git-mapping-plot3d.gmt.now.sh/api/generated/pygmt.Figure.plot.html#pygmt.Figure.plot.