Inexact representations

[willtebbutt]

Currently admit Integer-valued quantities to be differentiated w.r.t.. This is problematic in that the derivative a function w.r.t. an input which is an Integer is not necessary itself an Integer (or faithfully represented by one), yielding inexact conversion errors. Two possible solutions to this are:

1. Move from Reals to AbstractFloats.
2. Allow the gradient of an Integer-valued quantity to be represented by a Float.