FInAT transition plan

[miklos1]

Outstanding tasks before switching to FInAT:

• [x] Make the whole test suite pass.
  • [x] Fix the rounding of FIAT tabulation matrices.
• [x] Optimise evaluation of quantities that are constant on the integration entity.
• [x] Code snippet efficiency for the Jacobian of affine simplices.
• [x] Structured code generation for VectorElement/TensorElement.
  • [x] Optimise away Kronecker deltas for coefficients.
• [x] Structured code generation for TensorProductElement.
  • [x] Sum factorisation for coefficients.
  • [x] Sum factorisation for arguments.
• [x] Update the UFC kernel interface.

Things to do last before the switch:

• [x] Clean up the finat branch in TSFC and FInAT itself.
• [x] Have a look at the generated code for different problems to spot potential regressions. (The FFC regression tests can help with this.)
• [x] Look for regressions in form compilation speed and the performance of the generated code.
• [x] Test correctness and performance for a few critical projects (gusto, thetis).
• [x] Test the upgrade path (should just work).

Things that can wait until after the switch:

• Structured code generation for EnrichedElement/MixedElement.
  • Sum factorisation for RTCF, RTCE, NCF, NCE elements and for the appropriate H(div) and H(curl) elements on triangular prism cells.
• Use FInAT elements in Firedrake function spaces.
• Retire tsfc.fiatinterface and the supporting routines.
• Optimise away Kronecker deltas for arguments.

[miklos1]

Moved "optimise away Kronecker deltas for arguments" to the backlog. Since sum factorisation of arguments and argument factorisation will happen in COFFEE, this point might be difficult to do.