Add functionality to ignore exposures

[Hoeijmakers]

We need to add a functionality somewhere that allows the user to mask out exposures.

This needs to happen at the level of the obstimes. We also want people to choose the exposures they want to exclude from the CCFs.

The proposed plan is to make system_parameters functions aware of an optional mask-column in the obstimes file, and return shorter arrays. During reading in of the orders, those exposures need to be deleted, too.

After that, we can give people the option at the end when the CCFs/DS are plotted, to select exposures to reject; so as to make the obstimes remove column automatically. A keyword in the config file should be able to turn all of this exposure-rejection on or off; so that people can easily compare results with and without masking.

[Hoeijmakers]

Doing this on the level of obstimes is not a great idea because obstimes is overwritten each time you would run read_data. Its better to do this on the level of the config file. All these system_parameters functions read obstimes in the same way, so that can be its own function def read_obstimes.... So it better be done in the config file that read_obstimes then has to check for any exposures that are flagged. This then needs to be implemented on the level of read_data, whenever the orders, wavelengths and telluric corrections are read in.