Li_eval(): avoid the "numeric::operator>(): complex inequality" exception.

Li_eval(): avoid the "numeric::operator>(): complex inequality" exception.

Basically the problem is that 1 + 0.0*I is equal to 1, but checking if
the former is positive results in the the "numeric::operator>(): complex
inequality" exception. This behavior is definitely inconsistent and should
be fixed.

As a work around teach Li_eval and convert_parameter_Li_to_H to explicitly
convert 1 + 0.0*I to 1 (and -1 + 0.0*I to -1, respectively).

[Alexei Sheplyakov: comment the code a little bit, otherwise it looks weird]