Simplify the Simulation interface, improve MMS verification, and clean up validation tests

[agzimmerman]

Simulation class:

• Simplified interface. Before, there was a pattern where many parameters were set with init while others were assigned after instantiation. Parameters can still be assigned after instantiation, but the default pattern now is to include all parameters also as arguments to init. This cleans up a lot of the test/user code.
• Handle solver parameters in the same way as other parameters, starting as an argument to init. Before, it was more complicated to understand which solver parameters were being used when using sub classes which redefined the parameters by redefining the solve method.

MMS module:

• Verify accuracy of each solution subspace separately; remove mixed error norm.
• Report mesh cell count and function space DOF count.
• Expose the strong form and Dirichlet BC's as arguments so that the defaults can be overridden, though only the defaults are currently being tested.
• Updated many verification tests to check H1 norm instead of L2 norm
• Simplified interface for passing simulation keyword arguments

Convection-coupled phase-change simulation class:

• Make the element more general, e.g. allowing easy use of Taylor-Hood style elements
• Update verification tests to use H1 norm for velocity and temperature
• Report mean pressure and velocity divergence.
• Simplified some interfaces, allowing more parameters to be passed as argument rather than having to use assign after init.
• More descriptive names for liquid smoothing parameter and pressure penalty factor.
• Removed some artifacts from the validation tests, making them much more readable.