adding DivTime, DivHour and DivDay and appropriate tests into file_utils and test_file_utils
refactoring timestr to tstr
adding previous and next for DivHour
putting all path calculation to top. removing from Reader classes.
Implementing DivObs as  a collector class for Diviner paths and a handle to get next and previous observations.
Cleaned up FileName and added child classes L1A, RDRx.
adding la1,rdrs, rdrr getters to DivObs and tests for that.
Removing from_tstr from readers. this should be done in DivObs now, where appropriate paths/fnames are generated.
Adding IOError exception handling to readers, specific L1A/RDRR_/RDRS_/NotFoundErrors are being raised. These are NOT being caught by DivObs as they are informative.
debugging changes in L1ADataFile.
wip