GENERATORS: Prototype for fixing frequency/phase direct control issue

This is part of debugging #9.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
GENERATORS: Update phase control test to reproduce new issue

This shows #9 with 100% reproducibility.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
GENERATORS: Fix frequency/phase direct control issue

In order not to burden these lightweight generators, the external user
will have to explicitly call ProcessScalar() after having changed the
frequency/set the phase.

Fixes #9.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>