Better handling of nans in data

[psavery]

Currently, we expect any nan values in the data to be masked out via the panel_buffer. If they aren't masked out, we encounter errors in some parts of the code. For example, when running calibration, we encounter this error:

ValueError: The model function generated NaN values and the fit aborted! Please check your model function and/or set boundaries on parameters where applicable. In cases like this, using "nan_policy='omit'" will probably not work.

We should make sure that different parts of the code work fine when there are nans in the data.

This issue was created due to #1753