Some fixes related to #118

This fixes the "main" bug from #118 (closed) where dnadna train (specifically the training config file) couldn't properly locate dataset files when they are relative to a dataset config file in a different directory than the training config file.

It also fixes an ancillary issue that came up there, where running dnadna train without being able to load any files in the dataset resulted in a crash with an unclear error message:

AttributeError: 'NoneType' object has no attribute 'n_snp'