Check whether models allocate

[dpo]

Currently, the NLPModel type is not parameterized by the types of the problem functions. That may cause type instabilities / allocations. We should add corresponding unit tests.

[tmigot]

We can use this function to test test_zero_allocations
https://github.com/JuliaSmoothOptimizers/NLPModelsTest.jl/blob/main/src/allocs_model.jl

[dpo]

See #36.