Gradient tabulation bug-fix for the trace element

This PR is made in response to a bug (detailed in GitHub FIAT Issue 7) that was discovered while testing the TraceError handling when asking for derivatives on a trace element. A small change was made in hdiv_trace.py to make sure dict keys are accurate and hence the TraceError is inserted properly.

I also made the unit test test_hdivtrace.py more thorough: it tests on 2D and 3D reference simplex cells and tests for TraceError instances where appropriate.