seisflows configure relative paths sets 'null' as a path

[bch0w]

When using seisflows configure with the '-a' option to set paths as absolute, paths that are originally set as null (which represents NoneType in YAML format) get set as full paths. For example

path_mask: null

seisflows configure -a

path_mask: /Users/chow/Work/scratch/null

Will need to edit the configure script to ignore values set as None and as 'null'

[bch0w]

Similarly, the error message that results from this (SeisFlows trying to read model files from an incorrect path) is pretty vague and can probably be made more descriptive.

  File "/home/ykaneko/Samriddhi/seisflows/seisflows/tools/model.py", line 103, in __init__
    self.fmt = self._guess_file_format()
  File "/home/ykaneko/Samriddhi/seisflows/seisflows/tools/model.py", line 658, in _guess_file_format
    assert (len(fmt) == 1), (
AssertionError: cannot guess model format, multiple matching acceptable formats found: []