keras model_file expects str instead of Path. fixes #9

[mbhall88]

I am aware that #10 already addresses this issue. However, the way that PR goes about fixing the problem in #9 (in my opinion) is not ideal.

load_model expects a str or "h5py.File or h5py.Group object from which to load the model". @rrwick handles the model files internally with the pathlib.Path class which is an obvious design choice as these objects are indeed paths and therefore the added functionality this class brings within python is something Ryan uses a lot in the code.

I don't think changing the function find_model to return a str instead of a Path to satisfy the single use of keras.load_model is warranted.

[mbhall88]

fixed in e053457. PR no longer required.