Search path for data files is inconsistent

[astroleo]

For some files (e.g. the one defined in SCOPE_MIRROR_LIST) a file in the working directory overwrites a system installation. For other files, e.g. the one defined in INST_FILTER_TC, only the system directory is searched and if it isn't there, a "ValueError" is thrown as the file doesn't exist (should be a file not found error...).

[oczoske]

This is presumably because INST_FILTER_TC permits the use of abbreviations like "Ks" from which a file name is created. We ought to remove that possibility and only use filenames.