Dollar and data usage

High-level **vegan** functions should protect users from needing to use `$` sign to access results. Usually there should be support functions to access necessary results. This commits removes some easy cases of `$`: in most examples it is used to access variables within `data.frame`, and can be replaced using `with()` structure. 

In one case there even was a default support function `residuals()` that is able to access the results.

In addition, `ordisurf` used prolifically `with()` although this was not needed: the function has `data` argument.