XRD block suggestions and feedback

[ml-evs]

Some feedback from @MJCliffe about next steps for the XRD block. Some of these move more towards a dedicated XRD app (which wouldn't be a bad thing), which we could build out for certain techniques.

Plotting options:

• show point coordinates in cross-hair mode #1262
• ability to hide points and only show line #1264
• make plot generally bigger #1265
• ability to stagger rather than overlap patterns #1266
• move legend outside plot by default (long file names ruin the layout) #1267

Many of these can be made as improvements across all blocks by enhancing the big general bokeh plot method

Longer term:

• ability to rebin data with fixed resolution (and compute background from that)
• automatic peak picking
• hkl on hover for theoretical patterns
• ability to mark a given CIF as the "real" one for a sample, rather than just comparison

[BenjaminCharmes]

I created sub-issues for the plotting options, but in the end I did it all in one go. So maybe it wasn't necessary!