Bug with upgrade to SymbolicUtils release 1.6 and 1.7

[jbcaillau]

Hi there; recent problem when calling Ipopt on an ADNLModel, which is what we do in OptimalControl.jl. Was working fine until last week, I would say (first ran into the problem one day ago). Attached is a snapshot of the error, seemingly connected to the sparsity pattern analysis of the Hessian by means of Symbolics.jl (and SymbolicUtils). The problem can be reproduced by running the basic example from OptimalControl.jl here.

[tmigot]

Added a 1.5 limit in #235 in the docs and test Project.toml, to be dropped as soon as we get back on track.