rename calculateDPCoA, calculateNMDS, calculateRDA, calculateCCA

[thpralas]

This PR makes the following naming modifications :

• calculateDPCoA --> getDPCoA

• calculateNMDS --> getNMDS

• calculateRDA --> getRDA

• calculateCCA --> getCCA

[TuomasBorman]

Thanks! Certain ordination methods that we use come from scater package. They use calculate* and run*, for instance, calculatePCA and runMDS. That is why we have to think about if we

1. Keep these unchanged
2. Keep calculate* and run* aliases
3. Replace calculate* and run* with get* and add* like done in this PR
4. Add get* and run* aliases also for scater ordination methods.

I have to think about this little bit