analytic derivatives for fisherfit

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2630 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
re-write fisherfit and remove its standard errors

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2631 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
clean-up after r2631 (fisherfit re-write and se removal)

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2632 edb9625f-4e0d-4859-8d74-9fd3b1da38cb