Opening file if already opened elsewhere can lead to weird runtime errors

[Vindaar]

If a file is opened in one place and then opened in another place, certain HDF5 functions can fail with some really hard to debug errors.

We should try to detect that somehow. Opening a file when opened by hdfview fails, because the file is locked. Can we lock the file so an additional open call fails?

[Vindaar]

Note:

If opening a file from multiple locations is desired, set the following environment variable:

export HDF5_USE_FILE_LOCKING=FALSE

This is also mentioned in the README nowadays here:
https://github.com/Vindaar/nimhdf5#threadsafe-hdf5-library--file-locks

Before closing this, investigate if there's a way to ask the library for "is the file locked?".