Add Haslam map with scale factor

[telegraphic]

It is common to use the Haslam 408 MHz map with frequency scaling, should add support for this.

Proof of concept

def generate_haslam(f, scale_factor=None):
    haslam = hp.read_map('haslam408_dsds_Remazeilles2014.fits')
    f0 = 408.0   # 408 MHz 
    if scale_factor is None:
        scale_factor = -2.55
    return haslam * (f / f0)**(-2.55)