Provide help strings for all DIALS commands, including the "optional" ones

[dagewa]

It is unclear how to run the "optional" DIALS commands. It should be possible to run any of these without any input in order to print a help string. In fact, this could be useful for the standard commands too. Basically, there should be a "help" button in an obvious place that in the simplest case runs the command without any input and prints the log message to a pop up help window. The help button could additionally have verbosity and expert level controls, to achieve the effect of running e.g. dials.refine -hhhvv