allow non-absolute paths for input to level2 converter

[observingClouds]

Fails: sounding_converter -i test_level1.nc -o test_level2_{direction}.nc -c config/main.yaml
Works: sounding_converter -I ${PWD}/test_level1.nc -o ./test_level2_{direction}.nc -c ./config/main.yaml

Error:

Traceback (most recent call last):
  File "/pysonde/bin/sounding_converter", line 8, in <module>
    sys.exit(main())
  File "/pysonde/pysonde/pysonde.py", line 131, in main
    input_files = h.find_files(args["inputfile"])
  File "/pysonde/pysonde/_helpers.py", line 164, in find_files
    filelist = expand_pathglobs(arg_input[0])
  File "/pysonde/pysonde/_helpers.py", line 188, in expand_pathglobs
    return expand_pathglobs(pathparts[1:], [Path(pathparts[0])])
  File "/pysonde/pysonde/_helpers.py", line 190, in expand_pathglobs
    assert pathparts[0] != "/"
IndexError: tuple index out of range