Complete proposed changes to gonum/floats interface

As detailed in
https://groups.google.com/forum/#!topic/gonum-dev/kOSNjAYGpPc

Changed sigs for most functions which mutated inputs to no longer
return values.  Also made Add and AddTo align with other arithmetic ops.
update examples and test

Some examples and tests were no longer sensible, and were removed.  The
other ones were changed to use the new Add and AddTo functions.
Add test for AddTo, 100% coverage

Add a test for new function AddTo.  It was covered except for the
panics by the example code.  This brings coverage to 100%
run go fmt
undo non-improvement to Max, Min, and Nearest

These changes didn’t result in a performance improvement.  Rolling them
back.
Add missing periods to docs

Two of the public doc comments were missing periods at the end of
sentences.  Fixed.
Remove unused returns & return variables

There was no need for many of these return statements, and many of the
return variable meanings were quite obvious.

In particular, the LogSumExp function wasn’t actually returning the
same variable.  The rest of the changes are mostly cosmetic.
Put returns back into [Op]To functions

Functions which always result in the same dot slice should return
slices, as agreed during discussion in gonum-dev.

I also added tests to make sure that the returned slices were the same
as the mutated slices.  Even though the coverage was reporting 100%,
the tests were not covering 100% of the returned values.
go fmt the code

what it says on the tin
Add returned slices for AddScaledTo and Cum*

Added missing returned slices for AddScaledTo, CumSum, and CumProd.

Also added tests for the returned slices.
Merge pull request #12 from gonum/simplify-interfaces-retry

Simplify interfaces, as outlined in https://groups.google.com/forum/#!topic/gonum-dev/kOSNjAYGpPc