Add discrete log bounds and revise bounds dialog

This PR adds the possibility to calculate discrete logarithmic bounds, e.g. via sp.update(bounds='log') or sp.update(bounds='symlog') for symmetric bounds around 0. This option has at least one boundary per power of ten (which would be equivalent to sp.update(bounds=['log', 1])) and the second (optional) number controls how many number per power of tenth exists. sp.update(bounds=['log', 2]) for instance results is something like [1, 5, 10, 50, 100, 500], etc..

If this second number is omitted, then we calculate it such, that we are around 11 boundaries in total.

Additionally, this PR fixes the default setting of colorbar ticks.

Closes #13 (closed)

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes