ENH: Add a TwoSlopeNorm to the norm_handler

[matthewturk]

I think that the code works, but I need suggestions on how to more easily expose this behavior. I'd also like to see how we could potentially describe setting the min/max to be mirrors; for instance, right now the ramp on either side of vcenter changes, but it would be nice to set it to the maximum extent. (i.e., vmax = -vmin or something).

[matthewturk]

I'd also like suggestions on how to expose this in the field plugins

[neutrinoceros]

I need suggestions on how to more easily expose this behavior

I'll quote myself from the Slack conversation

How about having set_norm accept a str as its norm argument (currently this must be a Normalize object), and arbitrary keyword arguments **kwargs passed down to the actual Normalize subclass underneath ?
We could add support for a couple Normalize subclasses that are already in matplotlib (like "twoslopes" -> TwoSlopeNorm and "asinh" -> AsinhNorm ). It would also be trivial to add more in the future, and the NormHandler class wouldn't need to be changed at all (I think).

[chrishavlin]

+1 for expanding set_norm functionality! I think that's a nice compromise that leaves a clear way forward for future development.

[matthewturk]

I can get behind the string key!

Shall we put this in (once y'all are OK with it) and I'll work on the string key in a new PR?

[matthewturk]

So I guess the consensus is, no, this needs to all happen at once? :)

[neutrinoceros]

I think so ?