Custom fonts, ex31, and better handling

This is not a bug but I noticed when running the tests I get this warning from ex31:

`gmt [WARNING]: Representation of font type not recognized. Using default.`

Now, when running it manually I do not get that error. Yet, the plot that is generated does not use the custom fonts LinBiolinum* and LinLibertine* ( you can see the text, using Courier I think, exceeds the legend).


The nightly build of the documentation and gallery show the correct fonts. Some searching led me to rediscover this line in docs/scripts/CMakeList.txt:

-C-sFONTPATH="${GMT_SOURCE_DIR}/doc/examples/ex31/fonts

being passed to **psconvert** when it makes all the PNG plots needed for the docs. While that is OK, I think perhaps we should pass `C-sFONTPATH="${GMT_SOURCE_DIR}/doc/examples/ex31/fonts` to **gmt begin** so that running the script stand-alone will produce the correct answer. But of course `${GMT_SOURCE_DIR}` is not a defined variable when you just run the script. Testing alternatives, I changed the first few lines of the script to

```
FDIR=/Users/pwessel/UH/RESEARCH/CVSPROJECTS/GMTdev/gmt-dev/doc/examples/ex31/fonts
gmt begin ex31 png C-sFONTPATH=${FDIR}
```

That worked, but is not particularly nice. Can we think of a better way to handle these cases? My first attempt failed because I was not in the ex31 directory when running the script so any relative path would fail. It would also be nice to say

`gmt begin ex31 png C-sFONTPATH=fonts`

meaning the subdirectory fonts in the current directory. But that would also only work if running from that directory since we need to detect the current directory. Perhaps custom fonts should be always looked for in ~/.gmt/fonts? If such a directory exists perhaps **psconvert** should automatically append the **-C** option? Alternatively, using custom fonts requires adding them to PSL_custom_fonts.txt so if that file exists we may wish to add that directory? Or perhaps the PSL_custom_fonts.txt file should have an optional `#FONTDIR=directory` comment that we can parse and use in **psconvert**?

Also, our [documentation](https://docs.generic-mapping-tools.org/dev/cookbook/postscript_fonts.html#embedding-fonts-in-postscript-and-pdf) says you can avoid having custom fonts in the PS by running a special **ghostscript** command that places outlines in the PostScript instead. This seems handy when using special fonts and you want to share the PS or PDF with a journal. Should we simplify this step with a new **psconvert** option?

Finally, the **C-sFONTPATH**=_fontdir_ is not particularly pretty either. Maybe one solution is to introduce a new **psconvert** option **-N**[_fontdir_][**+o**] that would let users specify relative or absolute paths to a font directory and optionally request conversion to font outlines?