No documentation of CrystFEL columns

[kmdalton]

The docstring for rs.read_crystfeldoesn't say anything about the columns in the dataset which it returns. I suggest the following descriptions as a start:

• XDET,YDET : internal detector panel coordinates
• s1x,s1y,s1z : scattered beam wavevector which points from the sample to the bragg peak
• I, SigI : intensity, uncertainty
• ewald_offset, angular_ewald_offset : the distance in cartesian space in reciprocal angstroms or polar coordinates in degrees between the observed reflection and the ewald sphere
• BATCH : the image number

[JBGreisman]

@kmdalton Do you think it makes sense to incorporate this into #216 because it already includes planned updates to the implementation?