Check for out-of-date basis sets

[DanHickstein]

This is a placeholder issue, following the discussion here: #260 (comment)

Basically, PyAbel will currently load basis sets without checking when they were generated. If a user generates basis sets with an old version of PyAbel, there is the possibility that these basis sets will be incompatible with a later version. Alternatively, the basis sets may simply be wrong. We could catch a bug in one of the basis set generation algorithms and have no way of pushing this improvement to users who have a huge stash of legacy basis set files.

One approach would be to check the date modified of the basis set files and make sure that this date is more recent than the date of the basex.py (or other method) file. This would ensure that the basis sets were always up to date. For users who intentionally want to use old basis sets (for example, they have generated ones that take a very long time to generate, and are comfortable that no improvements have been made to the basis set generation code), we could allow a keyword argument to be passed to the transform methods that allows old basis sets to be used.