add numerous tests. Fix loadframe standalone file
- add numerous self-tests
- enhance behavior of loading functions to be tolerant of missing files, which is useful for loading a single data file. This was an early feature of PyGemini now brought here.
- fix bug with loadframe() that required adding "./" in front of filename in same directory pwd
Dev note: test coverage ~ 50% due to a lot of raw data loading functions that are rarely used.