Support derivatives of maps

[dlfivefifty]

I like overriding ctranspose so that

m = LinearMap(5)
m' # returns `ConstantMap` or something

[dlfivefifty]

I think this should be done via DualNumbers.jl for scalar maps. Maybe ForwardDiff.jl for Jacobians?

Should m' give the Jacobian in general?

[daanhb]

There is some work to do on maps. Currently, Jacobians are not implemented for any map other than a linear one. This syntax looks good to me, it is not unlike abusing ctranspose to compute the derivative of a function.

[daanhb]

Jacobians for some maps are functional now in the development branch. The idea is that the jacobian of a map returns a new map. There is now a ConstantMap and a ZeroMap to support this for affine maps.
I believe the development branch has been made 0.7 compatible by @vincentcp also, in the meantime.

[dlfivefifty]

Ive been using master on 0.7 without issues

[vincentcp]

Yes, I merged .7 compatibility with master.

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[daanhb]

Okay, I guess this issue can be closed then. Support for derivatives of maps is not complete, but the framework is there and it is clear (hopefully).