Additive shear bias

[elikrause]

Implement placeholder parameterization for additive shear power spectrum (Eq. 19 in https://arxiv.org/pdf/astro-ph/0506030.pdf) and interface with CCL methods for Hankeltransform to calculate additive shear contribution to shear correlation functions.

[marcpaterno]

We currently have support for a multiplicative bias, but not for an additive bias.

@tilmantroester, @jablazek, and @elikrause: we’d like your input on whether this issue still relevant. Have the necessary underlying tools in CCL been written?

Or is it better to close this issue, and wait for someone to open an updated one at the appropriate time?

[tilmantroester]

I had hacked together an additive shear systematic in old firecrown to run KV450. That would be in example of a statistic-level systematic, in that a template is added to the predicted two-point correlation function.
Having something like this as a showcase for how to implement a statistic-level systematic might be useful but I can't think of a specfic application outside of reproducing the old KiDS results off the top of my head.