Resolve "Improve import time"

This MR speeds up the time it takes to call import dantro by about 50% (on my machine: from 2.5s down to about 1.2s).

This is achieved by delaying the imports of seaborn, sympy, and networkx, which are the most costly (see #249 (closed)). This required some minor restructuring, but nothing too big.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted Existing tests pass
• Documentation extended or updated (in code)

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved

Related issues

Closes #249 (closed)