Run `normalise` as a part of `append`

[tomwhite]

The normalise command is really an internal implementation detail of append. Users should be able to just run the append command and the VCZ will be normalised if necessary.

[tomwhite]

We can just compare contig/position/allele fields to check if we need to normalise. Comparing these is relatively quick (once sgkit-dev/vcztools#356 and related issues are done).