In-Place Operations

[willtebbutt]

Although when propagating derivatives through code, there is little reason to want to support in-place operations, it is perfectly possible that a user may wish to write a routine which uses in-place operations and evaluate it without recording all of the stuff required for differentiating through the code. It seems dumb to insist that in-place operations are not performed; they should be supported with the proviso that if you want to propagate derivatives through code which includes this stuff, you will lose the performance benefits associated with in-place operations.

Furthermore, at the point where we support FMAD, in-place operations become a completely sensible thing to do (I think).

[willtebbutt]

This isn't going to happen. See #81 for details.