#93 Add imaginary part of spherical harmonics

[Erik Hänel]

For reasons of completeness, NumeRe should also know the imaginary part of the spherical harmonics, e.g. through imY(l,m,theta,phi).

Analysis:
The following function has to be implemented with sin(m*phi)

value_type parser_SphericalHarmonics(value_type vl, value_type vm, value_type theta, value_type phi)
{
    if (isinf(vl) || isnan(vl)
        || isinf(vm) || isnan(vm)
        || isinf(theta) || isnan(theta)
        || isinf(phi) || isnan(phi))
        return NAN;
    int l = (int)fabs(vl);
    int m = (int)vm;
    if (abs(m) > l)
    {
        return NAN;
    }
    else
    {
        return sqrt((double)(2.0*l+1.0) * parser_Faculty(l-m) / (4.0 * M_PI * parser_Faculty(l+m)))*parser_AssociatedLegendrePolynomial(l,m,cos(theta))*cos(m*phi);
    }
    return 0.0;
}

Implementation:
Implementation was done as proposed by the analysis.

Documentation:
Function was added to the documentation.

Tests:
Not needed as direct derivation from tested function, where only one standard function was changed.