Feature Request: Allow users to upload slope for tide correction.

[2320sharon]

At many beaches the slope is not constant across the entire beach and can be fluctuate across the span of the time series. To account for this users should be able to upload their own beach slopes that can be specific to different regions on the beach or specific to different time periods. To make this easy users should be able to upload a CSV file that contains the slope for each transect and optionally a time that the slope is most associated with it.

Example 1: Slopes Specific to Each Transect with Specific Times

In this example CoastSeg will use the slope value that's closest in time to the time the shoreline was captured to perform tide correction. For example, for transect 2 slope 0.04 would be used for a shoreline captured 2021-04-07 or earlier, but for a shoreline captured 2021-04-08 or later the slope value of 0.03 would be used.

Transect ID	Slope	Datetime
1	0.05	2021-04-05 14:49:58+00:00
2	0.04	2021-04-07 14:49:58+00:00
2	0.03	2021-04-08 18:49:58+00:00
3	0.02	2021-04-06 14:49:58+00:00

Example 2: Slopes Specific to Each Transect

For this example the slope for each transect will be used through the entire timeseries.

Transect ID	Slope
1	0.05
2	0.04
3	0.02

[2320sharon]

@dbuscombe-usgs @mlundine when you plan to do tidally correction with time varying slopes what format will your data be in? What would be easiest to upload with CoastSeg?

Would it be better to have a format like this? If the End Date column is empty I could always assume it means use this value to the rest of the time series.

Transect ID	Slope	Start Date	End Date
1	0.05	2021-04-05	2021-04-10
2	0.04	2021-04-05	2021-04-07
2	0.03	2021-04-08	2021-04-09
3	0.02	2021-04-06	2021-04-10