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This reverts commit e99747f6.

Apparently some internal JuMP logic depends on the bridges, preventing e.g.
single-variable objective functions from working with GLPK (and maybe other
solvers). Dodging that is easy, e.g. you can use `@objective(m, Max, x+0)`
instead of one that contains just `x`, but kludges like this should not be
required in the code.

the tests might have been kindof a warning sign

Among other, this fixes a ~5-20% performance regression for FVA.

(we do not use them at all, which is likely gonna hold for some time)

...and finally allow the whole FBA part to be successfully and easily separated
into its own efficient single-threaded step.

Also implement `looks_like_internal_reaction` for convenience

Also closes #427

Closes #419

fix tests

fix test

this allows sneakiness in linear optimization

Closes #391

@stelmo format

change name of function

change name of function

closes #405