Commonnexus

- `tools.normalise.normalise` now accepts a `remove_taxa` argument, making
it easy to remove taxa from a NEXUS file in a consistent way.

- Make NEXUS content created by `commonnexus` simpler to parse and thus
`commonnexus normalise` more useful as a tool to prepare NEXUS input for
other tools.

- Dropped py3.7 compatibility.
- Allow unquoted content for SYMBOLS sub-command of FORMAT.
- Warn (or raise Error) when duplicate character names are specified for a MATRIX.

- Cache translate mappings in TREES block to speedup translation of multiple trees.

- API: Added `Tree.newick_string` property.

- Fixed bug where the Nexus.cfg was not consulted when reading CHARACTERS MATRIX.
- Keep state labels when normalising a NEXUS CHARACTERS block.
- Better support for Morphbank-style references in NOTES.
- CLI: Support dropping characters by number.
- CLI: Support batch-renaming taxa via lambda function.
- CLI: Added `split` command to split Mesquite's multi-block NEXUS.
- CLI: Added `taxa --describe` option, which is particularly useful with Morphobank NEXUS.
- CLI: Added `trees --rename` option.
- CLI Backwards incompatibility: `characters --multistatise` now requires an argument for
the `--multistatise` option.