Move CopyModelUtils and ModelAsserts to mastodon core.

[maarzt]

Mastodon git has two utility classes that are probably of interest for all mastodon projects so the should be moved to the mastodon core. These classes are:

CopyModelUtils
ModelAsserts

TODOs

• Open a PR on Mastodon core
• Make the CopyModelUtils more flexible
  • Allow to rename cells (for example prefix)
  • Allow to provide a transformation for cell position
  • Whether or not to copy tags
  • Allow to rename tagset names
  • ...
• Change the format of the modelToString to reduce the duplications in the string.
  The format could be:

Spot(...) spot tags: <list of spot tags>
  -> link to Spot(...) link tags: <list of edge tags>
  <list all outgoing edges>