Add CosmoPower P(k, z) emulation

[itrharrison]

The current CosmoPower implementation only gives acess to the network for CMB C_ell. Seeing as e.g. my current ACT x DES runs are annoyingly slow (>2 days) and the time for each likelihood evaluation is dominated by CAMB:

[soliket.cross_correlation.shearkappalikelihood] Average evaluation time for soliket.cross_correlation.ShearKappaLikelihood: 0.228822 s  (9 evaluations)
[camb.transfers] Average evaluation time for camb.transfers: 0.625354 s  (9 evaluations)
[camb] Average evaluation time for camb: 0.962215 s  (9 evaluations)
[soliket.ccl.ccl] Average evaluation time for soliket.ccl.CCL: 0.0253867 s  (9 evaluations

I will implement the P(k, z) emulation in our CosmoPower Boltzmann Theory.

[defjaf]

@itrharrison I think we talked last summer about possibly using Harry Mootoovaloo's emuPK, documented in our arxiv:2005.06551 and arxiv:2105.02256. Would this be worth trying or would two emulators be too chaotic?