Make function registration local

[philipbarrett]

Registering user-defined functions appears to be a global action. This can lead to some undesirable outcomes. For example:

function foo( A::Float64 )
  f(x) = exp( A * x )
  f1(x) = A * exp( A * x )
  f2(x) = A * A * exp( A * x )
      # Function to register
  JuMP.register(:f,1,f1,f2)
      # Registration
  ####
    # Optimization problem here using f
    # Leads to some return statement
  ####
end

# Session code
use1 = foo(2.0)
    # Works fine
use2 = foo(3.0)
    # Gives error "Operator f has already been defined"
use3 = foo(2.0)
    # Also gives the error(!)

Here I want the symbol :f to register a locally-defined JuMP function. But it appears to be global and immutable. So I'd like to request the following features:

1. If possible, make function registration local.
2. If this isn't possible, add methods to: a) check that a function is registered, and b) de-register a previously-defined function.

The example above seems like a pretty important one. For starters, it is a way to use one function (here foo) to solve similar problems with differently parameterized objective functions. In addition, the current set-up leads to an error when calling the same function twice (see use1 and use3 above) with no apparent way to check for this (aside from a try-catch block).

Hopefully this isn't too hard to add, and it seems like the payoff would be pretty large. Thanks!

PS. This is based on my similar stackoverflow question here: http://stackoverflow.com/questions/38774351/how-to-check-if-a-user-defined-function-is-already-registered-in-julia-jump

[mlubin]

This is a prerequisite for #808. Wouldn't expect it soon though, unless someone steps up and submits some PRs.