Modify the length function to provide lengths of sites, datasets, &cetera.

Currently, because of the structure of a neotoma2 object using the `length()` function only returns the number of sites.

There are several options here.  One is to have something like `length(x, 'datasets')`, or we could do `ndatasets()`. 

I think that the base `length()` method does not allow other parameters, so that may not be an option.