Automatically determine Q range when using kinematic constraints with euphonic-powder-map

[ajjackson]

Currently I find that when generating kinematically-constrained powder maps the Q range has to be set by trial and error or by looking at the a QECoverage plot from Mantid. I would expect that 95% of the time kinematic constraints are used people will want to see the whole accessible range.

In principle we can calculate this pretty easily, so it might as well be done in the CLI tool if no --q-min or --q-max is set.

Technically that's an API-breaking change to a default setting, but it's hard to see how it would hurt anyone. (There's always a relevant XKCD.)

[mducle]

The default 0 for --q-min is probably ok. The default of 3 for --q-max is quite arbitrary though and would be nice to change it to match the kinematic constraints if those keywords are given. As for the API breaking... let's see if anyone complains...