solidThermo: Add rhoThermo interface

The old fluid-specific rhoThermo has been split into a non-fluid
specific part which is still called rhoThermo, and a fluid-specific part
called rhoFluidThermo. The rhoThermo interface has been added to the
solidThermo model. This permits models and solvers that access the
density to operate on both solid and fluid thermophysical models.

etc/codeTemplates/dynamicCode/fluidThermo

@@ -22,7 +22,7 @@ type
 typeRenamed
 (
     hePsiThermo psiThermo
-    heRhoThermo rhoThermo
+    heRhoThermo rhoFluidThermo
 );
 
 energy