Spherical Convolution operator

Hi Martin,

I used your apply_spherical_convolution function as a template to create both

• a function to create convolution kernels from functions within LMSpaces: get_conv_kernel_from_func
• two operators that perform convolutions on spherical fields, one taking a kernel directly , one creating it from a function (SphericalConvolutionOperator, SphericalFuncConvolutionOperator)

I tried to enable git-praise attribution of the code snippets to you by first committing your function under your name, but it does not seem to have worked.

Could you please take a look at the code? Is desirable to force the input domain to be a DomainTuple?
I also added a small demo, which seems to work fine.

Thanks!