Transformations to and from vectors

[mohamed82008]

Currently constraints are assumed to return a vector. We need functions to transform to and from vectors. For example to support constraints on sparse matrix valued functions or functions of structs.

[mohamed82008]

Relevant: https://github.com/invenia/ParameterHandling.jl

[mohamed82008]

PR to handle sparse matrix to and from a vector in ParameterHandling.jl JuliaGaussianProcesses/ParameterHandling.jl#21.

[mohamed82008]

Can also provide Nonconvex.flatten which calls ParameterHandling.flatten and handles arbitrary structs using NamedTupleTools throwing away the non-reals.

[mohamed82008]

Automatic flattening and unflattening when x0 is not a vector would be cool.