Detrending flags

[michael-petersen]

This PR promotes totPow and totVar to bools and sets rules for when both are set.

Bigger questions:

• Is this an issue for backward compatibility (and does it matter)?
• Is this our long-term detrending strategy?

[The9Cat]

While sensitive about changing the behavior of an mSSA configuration feature "on the fly", these flags are undocumented, experimental parameters used in a non standard way. Their existence is the toggle unlike other switches which are boolean. So promoting these to boolean with readthedocs and pyEXP dostring description seems sensible, if the consensus is that we keep them. I would rather keep them then remove them. So I'm in favor of @sophialilleengen's proposal. We might consider adding "info" messages confirming the selection when they are used to make the consequences of their choice clear to the user in real time.

I would like to see an implementation of a more general facility detrending options. I'm thinking of a class that provides a detrending and retrending functors for each channel that allow a general time dependent rescaling that a user might define from Python. However, developing this will require additional thought and testing.

[The9Cat]

This all looks good. I will open the desire for a more general detrending functionality as an issue.