enabled writing .DM with two different DM

Nils Wittemeierrequested to merge
nils.wittemeier/siesta:multi-DM-write into dev

Similar to !400 (merged) this is a preparatory step for SIESTA-BdG.

This MR adds functionality to store two DM objects in the .DM file. In the case of SIESTA-BdG, this will be the normal and anomalous DM.

How do you think reading should be handled? In my opinion restruct_Data should take care of splitting the DM and ADM parts, but I'd be happy to hear your thoughts on this one.

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