Shrink gmtmex.c by growing gmtmex_parser.c to be included in GMT core

**Description of the desired feature**

The gmtmex._extension_ library with the mexFunction needs to be built using the user's MATLAB version. Hence, we want to move as much of the code in gmtmex.c into the gmtmex_parser.c as possible to keep things simple. It seems to me that the ultimate mexFunction would just be something like this:

```
#include "gmtmex.h"
void mexFunction (int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
gmt_mexfunction (nlhs, plus, nrhs, prhs);
return;
}
```

Does gmtmex.c really need more in it, @joa-quim if we just place the rest in gmt_mexfunction and stick in the gmtmex_parser.c in libgmt?