Re-enable read-only flag on output of `depstrip`

[OndrejMarsalek]

The read-only flag here:

https://github.com/ceriottm/i-pi-dev/blob/f5817d977fc4f8ead76b768d90fcb2357ceabe05/ipi/utils/depend.py#L690

is generally a good idea, as the output of depstrip is a bare Numpy array and should not be modified, as that bypasses the depend mechanism. However, as it was not enforced so far, there are places in the code with assignment to the output of depstrip. Thus, some testing and fixing is needed before this goes in master.