start of configurable mesh processor support

[AndrewAnnex]

This is a mostly work in progress PR so it's not ready to merge yet. Hope to get reviews with suggested fixes.

Partially adds delatin and martini mesh processing methods from work done in https://github.com/AndrewAnnex/monticello and elsewhere by https://github.com/kylebarron/dem-tiler.

also adds bits to the cesium default viewer to allow swapping between the methods but I was seeing cached tiles from different modes as I swapped between them so maybe some other changes are needed in the front end to make changes cleaner.

Martini tiles are not working well at all currently which I think mostly has to do with the tile edge merging stuff so I've disabled parts of the TerrainGenerator for Martini currently.

Delatin seems to work okay.. but maybe having similar issues to Martini with edge merging?

other todos:

1. make the z axis error more easily configurable via the query url
2. add vertical exaggeration (really should be in the front end only)
3. ensure I am using Martini correctly, as it is not a drop in replacement for delatin
4. make tile size configurable (eg 512x512 etc)

[AndrewAnnex]

note I did not add pymartini as a dependency yet due to poetry issues...