Merge remote-tracking branch 'atgeirr/master'
Add early foundations for compressible flow solver

This is highly incomplete and refactorings are likely.
Merge remote-tracking branch 'atgeirr/master'
Merge remote-tracking branch 'bska/master'
Move buildAllCells() to anon. namespace.

This avoids diagnostics like "no previous declaration for ...".
Ignore 'ImpesTPFAAD' test program.
Include <cassert> for assert() macro.
Don't check input values.  Defaults are fine.

While here, also return zero, which although not strictly needed,
nevertheless indicates to the reader that we're exiting successfully.
Include proper file header.
Distinguish fluid from geology properties.

Most of class ImpesTPFAAD deals (or will deal) with fluid properties
with only a token nod to the geology--mostly in the form of derived
quantities such as transmissibility and pore volumes.  There is no
need to conflate the roles of fluid interface and geology interface
for this purpose.
Variable name change.
Add subset() free function.

Also add overload for UpwindSelector::select() taking constants (not AD variables).
Use subset() for simpler code.