Only document the long-form arguments #473

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