Add flag to turn off dolfin-adjoint

[finsberg]

Currently pulse will try to import different objects from dolfin-adjoint and fall back to dolfin if dolfin-adjoint is not installed. However, it might be cases where you don't want to use dolfin-adjoint even though it is installed. To solve this we can let the use specify a flag (i.e an environment variable) that will use dolfin even though dolfin-adjoint is installed.