New Alias System: Add the private _to_string function

[seisman]

Part 1 of PR #3238

[seisman]

mapping=True means using the first letter of a long-form argument as its short-form argument. For example, resolution="high" will be mapped to resolution="h". If we don't support mapping=True, lines 112-123 can be simplified to a few lines:

                if value not in mapping and value not in mapping.values():
                    raise GMTValueError(
                        value,
                        description="value for parameter {name!r}" if name else "value",
                        choices=mapping.keys(),
                    )

but without mapping=True, we need to write something like

mapping={"full": "f", "high": "h", "intermediate": "i", "low": "l", "crude": "c"}

when calling the _to_string function.

[weiji14]

I think better to be explicit (use the full mapping {"key": "value"} dict), and just have one way to parse things. We can use the first letter for resolution, but for others like interpolation/-n, where the mapping is {"b-spline": "b", "bicubic": "c", "bilinear": "l", ...}, it doesn't work.

[seisman]

I've removed mapping=True in commit c499aab.

[weiji14]

This looks ok to me, was there anything else you wanted to add?

[seisman]

This looks ok to me, was there anything else you wanted to add?

Need to wait for #3985