Enhance histogram handling facilities

[johannes-mueller]

#217 will drop rebinning mean stress transformed histograms. That will have a couple of consequences

• Information about mean stress will remain in the resulting histogram
• The resulting histogram will have the same number of bins as the input histogram. So a 32x32 load histogram used to be rebinned to a 32 bins. Now it will have 32x32 = 1024 bins, leading to significantly more computing demand downstream

That creates a new demand for more rebinning convenience: