Handle partially frozen and constrained atoms
Atoms frozen along some, but not all dimensions would still be moved
along all dimensions by constraints. Now such dimensions are frozen.
Note that the initial configuration might not obey the constraints,
which leads to conflicting demands of freezing and constraining.
Partially frozen atoms in the initial configuration will still be
constrained along all dimensions (but will be frozen during the run).
Fixes #1960.
Change-Id: Ic4d43a2840fabc084aec4237abf6a589eaa72f37