TwoSlopeNorm now auto-expands to always have two slopes

In the case where either vmin or vmax are not manually specified to 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.

Previously vmin and vmax were clipped at the center point, which caused issues when displaying color bars.

This does not affect behaviour when vmin and vmax are manually specified by the user.