Support time range and only combining some blocks in combine_h5

[nilsdeppe]

Proposed changes

This is useful for visualizations since getting tetrahedral connectivity to work is a lot easier with a single h5 file. These changes are part of a bigger pipeline support where you can combine some blocks into a new H5 file, then run a new bit of python code, generate-tetrahedral-connectivity, and then more easily to volume renderings in ParaView.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments