Allow for discontinous shorelines to be mapped

[2320sharon]

In the original version of coastsat shorelines have to be continuous, which can result in the extracted shorelines having a stitching effect to combine multiple discontinuous shorelines together as shown in the image below. Coastseg should allow for multiple discontinuous shorelines to be extracted from a single image without artificially stitching the shorelines together.

Here are a few ideas to start with

1. Allowing multiple reference shorelines to be uploaded.
2. Informing the users to expand the reference Shoreline buffer
3. Modifying the algorithm that creates the extracted shorelines to create multiple discontinuous shorelines

[dbuscombe-usgs]

closing this as a duplicate of #163