_fix_spcat(x) does not differentiate between lowercase and capital letters

[cixue]

In file_handling.py, quanta beginning with letters are all treated as if the letters are in capitals.

However, according to CDMS,

Quanta between –10 and –19 are shown as a0 through a9. Similarly, –20 is b0, etc., up to –259, which is shown as z9. Quanta between 100 and 109 are shown as A0 through A9. Similarly, 110 is B0, etc., up to 359, which is shown as Z9.

Need to handle lower cases for negative quanta.