Add squirrel tests

[smpark7]

Merge #18 before this PR. Fixes #16

Summary

Adds CI test coverage for all objects in squirrel. Squirrel was previously indirectly covered by Moltres CIVET testing. But many objects were not tested for their functionality, thus we only caught build issues.

Contains fixes for several bugs or missing functionality that I deemed essential.

Fixes missing test function calls in DiffusiveFluxBC and InterTemperatureAdvection.

Fixes missing material property input params for rho and cp in PostprocessorTemperatureInflowBC, VelocityFunctionTemperatureOutflowBC, DGFunctionTemperatureAdvection, and VelocityFunctionTemperatureAdvection.

Adds PostprocessorVelocityFunctionTemperatureInflowBC for an inflow BC that accepts a velocity function. The inlet temperature postprocessor also accepts constant scalar values.

List of Changes

• New tests covering all aux kernels, BCs, DG kernels, interface kernels, kernels, user objects, and vectorpostprocessors.
  • Tests basic functionality and Jacobian correctness.
• Delete simple_diffusion test which seems to be a copy of a MOOSE test
• Add _test premultiplier to:
  • DiffusiveFluxBC residual
  • InterTemperatureAdvection heat_source residual contribution
• Add PostprocessorVelocityFunctionTemperatureInflowBC
• Add missing rho and cp material property input params in PostprocessorTemperatureInflowBC, VelocityFunctionTemperatureOutflowBC, DGFunctionTemperatureAdvection, and VelocityFunctionTemperatureAdvection.