Linear EOS bugfixes

[cbegeman]

The following bugfixes are provided related to the linear equation of state option:

• Prior to this PR, the linear equation of state option only updated in situ density and not potential density. Now we compute both.
• In situ density is now a function of pressure whereas it was previously pressure-independent.
• If the linear equation of state and constant coefficients (fixed_alpha) are both on, there is no need to compute the nonlinear terms. An if-clause is now used to bypass these computations.

[cbegeman]

@katsmith133 Do you want to review this PR? It's not the most interesting development, but simple enough for someone who just started looking at the code. @qingli411 Perhaps you could be the second set of eyes here?