fix unconstrained assembly.solve()

[greyltc]

Fixes #591

[codecov]

Codecov Report

Merging #592 (48d7800) into master (4c77c87) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #592   +/-   ##
=======================================
  Coverage   94.26%   94.27%           
=======================================
  Files          29       29           
  Lines        6348     6353    +5     
  Branches      675      676    +1     
=======================================
+ Hits         5984     5989    +5     
  Misses        226      226           
  Partials      138      138           

Impacted Files	Coverage Δ
cadquery/assembly.py	90.54% <100.00%> (+0.09%)	⬆️
tests/test_assembly.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4c77c87...48d7800. Read the comment docs.

[adam-urbanczyk]

I reworked to raise a ValueError with a clear message. Anything against @greyltc ?

[greyltc]

Yeah, it's very likely the user has made a mistake if they call solve() without setting up constraints.

I can imagine a case where it might be convenient for solve() to be a noop (or just throw a warning), like if you're troubleshooting constraints and you've commented them all out or something.

I'm fine with leaving it as an error though.

[adam-urbanczyk]

I'm on the being explicit side on that one - solve only makes sense with constraints.

[adam-urbanczyk]

Are you fine with merging @jmwright ?

[jmwright]

+1