Ready for version 0.2.0 release

Added correlation selector #27
Fixes field selection bug
Adds ragavi-vis for visibility plotting using datashader #19
Vis plotter colourisation possible over scan and antenna except baseline #22
Some rtd documentation within
Field selection in ragavi-gains now MUST be done through comma-separated values rather than space-separated.
Logging done to terminal and to file #23
Introduces ymin and ymax for ragavi-vis to remedy #41

Current interface for ragavi-gains

ratt-ru / ragavi