Add additional tests

[stelmo]

• [x] metabolite connectivity
• [x] unbounded flux in default medium

[exaexa]

I think this is partially done and partially addressed by #50 ?

[exaexa]

(cc @Fl-Sch pls confirm if you can :] )

[vm-vh]

Yeah these functions were both included in #50 (see the last 2 functions in Consistency.jl)

[exaexa]

@vm-vh ok perfect, thanks for confirm :]