Add metadata to settings

Settings (internal single scalar values which can be read/written)
need to be marked as such.

Also add a function to test if a supplied value is correct for a
given setting (or more generally, correct for a given argument in
a command).
Add first cut LS30::Model

This 'Model' essentially stores all the settings of an LS-30 device
and can set/get those settings, proxying to an upstream (presumably
real) device if one has been set.
Add test script for LS30::Model

Might as well make a start on automated testing of this package.