Seeding improvements

[claresinger]

1. Shuffle logic: e.g. don't need to shuffle if only one type of seed particle or if the number of super particles to inject is equal to the number of possible seeds.
2. Example comparison from literature (maybe https://doi.org/10.1098/rsta.2012.0086), some other candidate papers listed in Time-dependent super-droplet sampling and modification #1331
3. Example of seeding in 1D and 2D domains
4. Parallelisation of the sampling logic
5. GPU support

[jtbuch]

I've made a PR addressing point 3. above in #1396