Improve import time by delaying package imports

This MR improves dantro's initial import time from ~1s down to ~0.1s 🎉

This is achieved by using dantro._import_tools.LazyLoader to delay the imports of many dantro dependencies: xarray, h5py, matplotlib, dask, dill.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted (stay exactly the same, as they should)
• Documentation extended or updated

• 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 #293 (closed)