PDEstimateDetectorResolution improvements

[peterfpeterson]

This was originally trac #11182.

To test: Make sure that the new functionality from the ticket is in place.

• It validates the input parameters on exec rather than in init
• It should be mentioned in CalibrateRectangularDetectors and GetDetOffsetMultiPeaks as well as link to them
• Make it work with a "metadata only" workspace
• Allow the user to supply the cetre wavelength

Special Note: This does introduce a small change to LoadEventNexus so doing some testing of unrelated functionality that requires event nexus files is a good idea.

[peterfpeterson]

It looks like I broke the usage test.

[wdzhou]

The new features work fine. The ticket is to be closed.

But the documentation of LoadEventNexus should be modified because the behavior of loading meta data only is changed, i.e., the number of spectra in output workspace is change.