General Description: If the NGD that has a PERIOD defined, but the
UCF doesn't, it can not be deleted or changed in the Constraint Editor.
If a new PERIOD constraint is created in CE the group is added to
the ucf but not the new PERIOD Timespec.
To resolve this issue open the UCF file in a text editor and add the desired
constraint. For Example:
TIMESPEC "TS_CLK" = PERIOD "CLK" 20 ns;