Add flag to turn off dolfin-adjoint

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.