feat: add config workflow for wavelength

[yucongalicechen]

          I wonder if the default should be `MoKa1`.  I am not sure which is the most commonly used.  Also, it would be good if it could be loaded from a config file.  In this way, if it is lab diffractometer, the correct one could be in the config file and loaded by default, but in a different lab, they could update the config file accordingly.

Originally posted by @sbillinge in #155 (comment)