determine_structuring_parameters/autostructure clunky to use correctly

[ardunn]

Written with the intention #150 is merged

determine_structuring_parameters (which also runs autostructure now) should work correctly using only the data given to the class inside __init__/from_file.

i.e.,

# don't need to set env var for it to run correctly
# should just run

datapath = BEEPDatapath.from_file("some_file")
params = datapath.determine_structuring_parameters()
datapath.structure(*params)

Or, identically,

datapath = BEEPDatapath.from_file("some_file")
datapath.autostructure()

Currently using determine_structure_parameters requires the raw filename and the filename of the parameters file, which changes based on BEEP_PROCESSING_DIR. Forgetting to set this will cause silent errors where diagnostic_available is mistakenly returned as false, leading to diagnostics not interpolated or summarized when the diagnostic actually is available.