Add DropAxis transformer

[cboulay]

Some operations may yield a tensor where one of the dimensions is always length-1. e.g., PCA with n_components=1, or BandPower with only 1 band defined, or Slicer to select only 1 channel. It would be good to have a node that removed an axis. There are a couple ways to approach this.

1. Squeeze out all Len-1 axes unless protected by a setting (e.g., iter_axis = "time" should not be dropped if we happen to get a single timestamp in a chunk).
2. Only remove a single axis. If its length is > 1 we could choose to either select the first element or to raise an error.

Thoughts?