Improve levels auto_range with proper symmetry

[juntyr]

The current implementation produces asymmetric levels when a divergence point is used, and is prone to rounding errors.

This patch

• adjusts both the levels start and end to be a multiple of the bin_width (relative to the divergence point)
• uses np.linspace instead of np.arange for better rounding

[FussyDuck]

All committers have signed the CLA.

[JamesVarndell]

This is a great improvement - many thanks for your contribution! I've made a small change (removed the now unnecessary math import), but happt to approve this.