add time increment setting

[michaelchin]

https://github.com/GPlates/gplates-app/issues/8#issuecomment-1120648666

Some of our models go from 250 Ma to 0 Ma, and so most people would probably want to see the frames in 1 Myr increments (the smallest time increment we make the files in). But for other models, e.g. 1000 Ma to 0 Ma, you might only want to see every 10 Myr or 100 Myr - otherwise you could be sitting there for ages!

@michaelchin, could every X Myr be downloaded from the server on the fly to then be played in the app? Would mean that potentially a lot less files are downloaded in some cases (especially for the billion year model).

I think we need this time increment setting and we need to deal with irregular time sequences. https://github.com/GPlates/gplates-app/issues/8#issuecomment-1120673672

[michaelchin]

already done in "Animation Settings" subpage