Add close method to openmc.StatePoint and update docs/examples for best practices

[pshriwise]

Movitation

A recurring issue in the forum is users running into the error

RuntimeError: Failed to open HDF5 file with mode 'w': summary.h5

when working with OpenMC's Python API. This is usually related to an open StatePoint file when OpenMC is being executed. The open HDF5 file handle prevents an overwrite of the summary.h5 file (auto-linked to the StatePoint object by default) and causes OpenMC to crash. This can also happen if OpenMC tries to overwrite a statepoint file with an open handle, so we can see this even if writing the summary file is disabled.

A couple of forum discussions related to this issue:

• https://openmc.discourse.group/t/plot-tally-result-when-using-distribcellfilter/1247/4
• https://openmc.discourse.group/t/error-in-opening-h5-file/288

Changes

This PR adds a openmc.StatePoint.close method that closes the HDF5 files related to the statepoint object. StatePoint file handles are most easily managed using context managers when extracting data, but there are times when these regions may become unreasonably large. The StatePoint.close method is meant to handle these cases.

To increase user awareness, I've also updated the examples to use context managers when possible and always close the StatePoint objects as a matter of best practice if needed.

[paulromano]

Good call on adding this method and updating the examples. Also, thanks for adding something to our long-neglected troubleshooting guide 😄

[paulromano]

Thanks again @pshriwise!