New Cheb emulator

[marcobonici]

Right now, we have two different approaches for emulation.

• The standard one, predicting the $C_\ell$'s on a multipole grid with all integer multipoles
• Emulation the Chebyshev coefficients and using them to interpolate on the required grid The latter has several advantages, but is also prone to numerical instability. The best thing is to make something intermediate:
• Emulates the $C\ell$'s on the Chebyshev nodes grid, between the chosen $\ell\mathrm{min}-\ell_\mathrm{max}$
• Evaluates the Chebyshev coefficients $a_{\ell m}$
• Interpolates as in the previous scenario

This will also requires adequate AD rules (in a subsequent task).

[marcobonici]

Opened new branch. Need to update tests and docs. After this, involving Marius.