instead, let's bit the bullet and add the few lines required for
regula-falsi-Pegasus method whenever the old RegulaFalsi class was
used. note that this leads to slightly different results for the
SPE5CASE1 flow test. I suspect that the old solvers behave in
unexpected ways, though...
Note that because the inverted functions are usually piecewise linear,
inversion can be done in a much smarter way.