Support for virtual datasets

• Adds a new NXvirtualfield class to implement HDF5 virtual datasets.
• Checks for virtual fields when loading HDF5 files.
• Adds a new nxconsolidate function and command-line script to consolidate NXdata groups from multiple files into one NXdata group containing a virtual dataset.
• Adds a new nxdir script to print a NeXus file tree from the command line.
• Adds support for qualitative color maps in the plot module, with alignment of integer values to the discrete color bands.
• Converts package to use the latest recommendations for package distribution, such as creating a pyproject.toml file to configure the build backend, moving metadata and requirements from setup.py to setup.cfg, and using setuptools_scm for version management.
• Improves PEP-8 compliance of code.