Compute the cross-power of two real functions for a single spherical harmonic degree.

## Usage

cpower = SHCrossPowerL (cilm1, cilm2, l)

## Parameters

cpower : output, real(dp)
The cross power of the two functions for spherical harmonic degree l.
cilm1 : input, real(dp), dimension (2, lmaxin1+1, lmaxin1+1)
The spherical harmonic coefficients of the first function.
cilm2 : input, real(dp), dimension (2, lmaxin2+1, lmaxin2+1)
The spherical harmonic coefficients of the second function.
l : input, integer
The spherical harmonic degree. This must be less than or equal to the minimum of lmaxin1 and lmaxin2.

## Description

SHCrossPowerL will calculate the cross-power of two functions expressed in 4-pi normalized spherical harmonics for a single spherical harmonic degree l. This is explicitly calculated as:

cpower = Sum_{i=1}^2 Sum_{m=0}^l cilm1(i, l+1, m+1) * cilm2(i, l+1, m+1).