add energy features to Diffractometer base class

[prjemian]

As requested, fixes #43 .

In #25, #26, and #42, an example was prepared to show how to connect energy from the local controls with the calc engine energy. The additional features were:

• allow for an offset energy
• allow for conversion of energy units to the keV used by the calc engine
• add a flag to allow/block automatic updates of calc engine when control energy changes
• direct command to bypass that lock yet keep the offset and units conversion

@ambarb, @stuartcampbell, @stuwilkins, @gfabbris : Your comments and suggestions are welcome.

[prjemian]

This is marked as draft since the examples in the documentation need to be revised to use the new code additions.

[prjemian]

@jklynch @MikeHart85 @mrakitin Might need to build the documentation to review

[prjemian]

@ambarb, @stuartcampbell, @stuwilkins, @gfabbris : Can you review this PR?
I'd like to merge this before the end of the month.
Latest documentation attached here as PDF: hklpy.pdf

[prjemian]

@jklynch @MikeHart85 @mrakitin Can you review please? See PDF (above) for build documentation.

[stuwilkins]

@prjemian Will look at this today.

[prjemian]

I'm ready to merge this today if there are no objections.

[ambarb]

@prjemian I had a quick look this morning and most of the changes look like ones we discussed as you were developing other things. Just wanted to follow up and say that some of these changes are really exciting (particularly locking on beam line E and using the EGU pv so people can drive the beamline as they are accustomed (eV or keV).