bug: another divide by zero error in design_optimisation.py

[drbenvincent]

File "../../bad/optimisation.py", line 89, in design_optimisation
    U = (1/nD)*np.ones(nD)
ZeroDivisionError: division by zero

Maybe zero designs are being passed in? Overaggressive culling of designs in the heuristic code?

This definitely happens when we forget to provide inputs to one of the design variables. So:

• [x] add validation checker of provided design variables before we build all the designs. Do this in DARC_Designs.__init__()
• [x] refactor DARC_Designs.__init__()

There may or may not be other things which cause this error.

Add some asserts:

• [x] design_optimisation() should error if there are no (or very few) in designs
• [x] add a logging.warning in DARC_Designs.refine_design_space() if there are very few designs being generated

[drbenvincent]

No designs being available was the primary (only?) cause of this error, and it is now being caught and warned about in multiple places. The problem was primarily happening when we had a mis-structured design space spec that we were asking for in the magnitude effect style design spaces. This is a separate issue, see #28.