Draft: All current mantle convection forms including SUPG forms, ApplyScaled kernel operation, constant Coefficients, some math helper functions

Closes #9.

This branch contains:

• a lot of additional forms (including some for testing purposes) for mantle convection and supg stabilization
• an additional kernel operation called applyScaled that is only generated if you set the "-sc" flag of the generate_all_operators script to true
• the ability to add constant coefficient symbols to an operator (defined in the constructor) with the "-cc" command line option
• added some math helper functions for the element diameter and SUPG delta function

There are a few inconsistencies that we need to take care of:

• There are now two shear heating forms and both do something different: tau(u) : grad(u) or tau(u) : eps(u) respectively
• tau(u) is defined as with - 1/dim div(u) I for both shear heating forms but for full stokes with -1/3 div(u) I, hence we need to make this consistent and either use the pseudo 3D form or 1/dim form for both 2D and 3D
• There are two supg diffusion forms called supg_diffusion and diffusion_supg one of which includes a hard coded delta function and the other accepts a fem function input