* Autodiff for abs

* Formatting, more tests for abs

* Cos autodiff; also a clarifying comment in the abs test

* Forgot cos.cpp

* Sin autodiff

* Again, forgot to add sin.cpp  :/

* Tan autodiff

* Minor formatting tweak

* Commit partial work on select backprop so I can run valgrind on the server  :/

* Fix boolean thingy so it works on Linux

* Autodiff for ceiling, convert (untested), floor, sign. Fix unit test for tan. Implement ceiling, floor in VM.

* Fix bug in abs/sign unit test ranges

* Add multiplicative inverse and square-root ops (needed for hyperbolic trig autodiff)

* Better formula for sqrt adjoints

* Autodiff for hyperbolic trig ops

* Forgot to add cpp files for hyperbolics

* Remove inv (don't need it after all); also formatting (oops)

* fix bug with Convert autodiff

* Autodiff for concat

* Restore the accidentally-commented-out unit test for abs

* Formatting

* Fix 'unordered_map.at' exception when Adjoints::Adjoints visit a node that has never been add_delta'd; remove workarounds for that bug

* Fix erroneous 'not implemented' docstring for op::Not

* Autodiff for sum

* Checking in broken support for replace_slice so I can test it with valgrind on the dev box

* Fix unit test bug with tensor initialization; formatting

* Implement replace-slice in CPU backend

* Autodiff for Slice and ReplaceSlice

* Tweak docs for ReplaceSlice

* Remove no-longer-needed cast of arg list to runtime::TensorView