ElemDiver interface, impl, and tests

I’ve added in an implementation of ElemDiver, and tests.
go fmt
fix test for DivElem when result should be Inf
fix test for nan comparison

NaN != NaN, oops
rename equalsNaN to same
change temp assignment to var

Changed lines with `temp := &Dense{}` to `var temp Dense`
Merge pull request #57 from gonum/ElemDiver

ElemDiver interface, impl, and tests