Moose Workshop: change heat capacity to density * specific heat

[pbehne]

Motivation

The Porous flow example that is the foundation of the MOOSE workshop training formulates the energy conservation equation using heat capacity instead of the more common formulation that uses specific heat and density. The workshop slides should be updated to use the more common formulation, which is what is actually implemented in MOOSE.

Design

Update workshop slides, replace all occurrences of "C" with "\rho c_p".

Impact

Improve consistency between the tutorial problem statement and its implementation in MOOSE, avoiding confusion.

[pbehne]

I'll do this, if assigned.