Daniel Jonathan Weigand

parameter.cache() will change an invalid cache to valid, even for parameters with `get_cmd=None`

2

### Steps to reproduce ``` p = Parameter("name", set_cmd=None) assert not p.cache.valid p.cache() assert not p.cache.valid ``` ``` p = Parameter("name", set_cmd=None) p.set(5) assert p.cache.valid p.cache.invalidate() assert not p.cache.valid p.cache()...