R cmd check import fix

Explicit import or importFrom were added to the NAMESPACE, and some of these files were moved from Suggests to Imports in DESCRIPTION. Flagged require() were commented in *.R files for imported packages.

This was all experimental and changes were made when R CMD check flagged a NOTE or a Warning. For instance, I do not understand why some packages could be left in Suggests, but others had to be moved to Imports, and why some require() could be left in R functions, but others had to be removed. 

One example (ordisurf) also needed a small edit.

This was a crazy thing to do, since this did not influence vegan functionality: everything worked like a charm without this change, but obviously in a "wrong way". The change was only made to satisfy R CMD check.

R Under development (unstable) (2014-09-01 r66505) -- "Unsuffered Consequences" was used in test.
We should probably do a `require("mgcv")` so that **mgcv** gets loaded in this example. This usage is fine as the package is in `Imports:`.