Linear regression in scatter plot cannot be overwritten from default

[CoryMartin-NOAA]

This is because of how the code checks to add the regression line or not:

It currently does (pseudo code):
if 'linear regression' in config.keys()

when it should do something like:

if config.get('linear regression', True):

Right now, the key is defined in the default YAML, so it cannot be turned off unless it is 'deleted' from the config. This is hard to do. It will be much easier to make the YAML and the code have a boolean key.

[danholdaway]

I think this is done now right?