Variable name change.
Add subset() free function.

Also add overload for UpwindSelector::select() taking constants (not AD variables).
Use subset() for simpler code.
Assert non-empty state quantities.

This avoids generating a null-pointer dereference for

   &s[0]

in case of s.empty().
Merge remote-tracking branch 'atgeirr/master'
Rename 'phase' parameter p->phase to avoid confusion.
Implement simple constructor for sparse, diagonal matrix.

This is, effectively, the same as MATLAB's

    spdiags(d, 0)

constructor.
Implement first cut at pressure system assembly.

Compile-tested only.