Precision does not work for error < 10^-10

[AndreaIdini]

setting err = 1e-10 the box wave function tests ok.
However, setting a lower error threshold implies false test. The numerov solution output is up to 10^-11 precise, but setting a lower precision does not put it in infinite loop. Probably the limit on bisection blocks the infinite loop, and the 0.1 mesh implies the limit on precision.

To be verified.

[AndreaIdini]

If not specified, calls cmath. Eventually this does not support overload.
std::abs funziona come cmath fabs, e cmath abs funziona in modo bislacco.

[AndreaIdini]

err is used extensively in Numerov routine right now, it should be made with more thought, cf. comments in #43