Read alm from a fits file.
In the fits file, the alm are written with explicit index scheme, index = l**2+l+m+1, while healpix cxx uses index = m*(2*lmax+1-m)/2+l. The conversion is done in this function.
Parameters : | filename : str
hdu : int, optional
return_mmax : bool, optional
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Returns : | alms[, mmax] : complex array or tuple of a complex array and an int
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