Refactoring for custom inputs

[mdp0023]

For each method, whichever previous files it relied on are now user defined parameters. If parameter is None, it uses the default path to that file. All file paths are set when the class is initialized.

Each method still checks if the user defined or default input exists and returns a value error if it cannot find one.

Other major change is how file names are created. Since all default file names are initialized with the class, the tool get_file_path() is a bit redundant, and file paths can just be set within the pygeoflood class.

[markwang0]

looks great! I made two small changes:

• initialize with dem_path=None so we can still initialize with no arguments
• updated run_fim_workflow to check if files exist with is_file() rather than is None

[markwang0]

sorry, just realized we can't initialize dem_path=None, because we need that to set all file paths when instantiating the class. changed it back in d668216