Clarify input/output filepath or dirpath

[spestana]

As mentioned in #34 , it is not always clear if a function needs a path to a directory from which to read or write files, or a filepath (or if both options are acceptable).

We can clear this up by making variable names more explicitly descriptive, and update documentation.

I don't believe there is any widely accepted standard used in python, so I propose something simple like:

• example_input_filepath for where a full file path is needed, in this example, for some input file
• example_output_dirpath for where a directory path is needed, in this example, to specify where to write output files

And for manipulating filepaths, we should use a package like os.path, e.g.: output_filepath = os.path.join(output_dirpath, output_filename)

I'm open to hear your thoughts on this @IanChiu-333 @etboudreau

[spestana]

This has been addressed with #36