fix dcs.DCS serialisation

[jhidding]

adds serialisation for the DCS class in cslib.DCS module

[lvkessel]

As I remember, there were three DCS classes in cslib, two of which were being used by cstool. Just to save myself a lot of searching: was this one being used at all, and if so, where?

BTW, I removed the unnecessary DCS classes in the big refactor of a few months ago. Because I don't want to mess up Annemarie's progress and because even the refactored version is still quite hacky, it still lives in a separate fork. For all practical purposes, though, this PR addresses a version of cstool/cslib/pyelsepa that is deprecated. If it doesn't fix a problem with the deprecated cstool version, there is no real point in merging.

[jhidding]

I think it does fix something. I couldn't get it to work otherwise.

[lvkessel]

Looks like you're correct: seems like the "DCS2" class was used for storing the ELSEPA results, and that's exactly what we want to serialize. The obvious question now is: why did it work without DCS2 in the registry?