update join model scores functions to allow both or one of the csvs to be passed

[2320sharon]

I updated these functions so that the user can pass one or both of the csvs with the model scores. I also renamed the columns from model_scores to 'classifer_model_score' and 'segmentation_model_score'

[dbuscombe-usgs]

Do we need 4 variables to make this really useful?
'Image_suitability_model_score'
'Image_suitability_model_threshold'
'Segmentation_suitability_model_score'
'Segmentation_suitability_model_threshold'

[2320sharon]

Alright I added the thresholds for the classification and segmentation models. I also updated coastseg to save the thresholds used to the classification csv file. You can find these changes on this branch in coastseg : https://github.com/SatelliteShorelines/CoastSeg/tree/add_model_scores