ENH: Castep custom species

CASTEP supports "custom" chemical species defined by their own pseudopotentials. These are basically different versions of basic chemical species which may have different assumed starting electronic configuration, be different isotopes of the same element, etc. They are indicated with ":". For example we might have C:12 and C:14. As it was, ASE would cause errors when parsing these lines as the colon is treated as a regular separator.
I've implemented support for these custom species. Now they are correctly parsed and interpreted, and if present they are stored in a castep_custom_species array - while chemical symbols keep working as usual. These are also automatically written out if present. I also reparsed a bit input and output in io/castep.py to include these changes and allow more flexibility in the possibility of future ones.