For dual porosity models, use PORV from the matrix part.
Also guard all conversion of 1-based to 0-based indexing to avoid potential undefined behaviour.
This commit refines the error message emitted if a particular
phase does not have a PVT interpolant--e.g., if the INIT file
does not provide PVT data. Previously we would generate a
message akin to
Region Index 0 Outside Valid Range (0 .. (size_t)-1)
which is not very helpful to users in diagnosing the issue. The
new message does at least indicate that there is no interpolant
for the accompanying phase and also emits the (1-based) region
index for context.
