Error when calling a ME_NeuralFMU without having defined a solver

[juguma]

When you call an ME_NeuralFMU

FMIFlux.jl/src/neural.jl

Line 1043 in 177045b

function (nfmu::ME_NeuralFMU)(x_start::Union{Array{<:Real}, Nothing} = nfmu.x0,

, for which you haven't defined a solver, you get an error
MethodError: no method matching isimplicit(::Nothing)

This is since in neural.jl, line 1290 the definition of the sensealg is based on the definition of the solver.
MWE is the script of #120 up to batchDataSolution (which fails).

If you simply leave the sensealg in such cases as nothing (e.g. modify line 1290 to if !isnothing(solver) && isimplicit(solver)), batchDataSolution runs through smoothly. However, I am not sure this is a globally valid solution (which would mean that both the solver and the sensealg are determined by heuristics of other packages!?).

[juguma]

Fixed by #126