Rename removeNA and fix its implementation

I've come to really dislike the use of caps in the name for `removeNA`, which I think is also too long. It's renamed `dropna` in this PR.

I also dislike the absence of a custom implementation of `dropna` for PDA's, so I added one.

I decided to make `dropna` only apply to DV's and PDV's because the operation doesn't make sense for higher-order tensors.

Looking through these changes, I realize that the iterators look really ugly now. Will fix them before this is merged.