TwoSlopeNorm now auto-expands to always have two slopes#
In the case where either
vmax are not manually specified
TwoSlopeNorm, and where the data it is scaling is all less than or
greater than the center point, the limits are now auto-expanded so there
are two symmetrically sized slopes either side of the center point.
vmax were clipped at the center point, which
caused issues when displaying color bars.
This does not affect behaviour when
vmax are manually
specified by the user.