Derivatives: task list

[JCGoran]

The below are all leftover tasks from #5 that were not mandatory for that PR to get merged.

• implement third order mixed derivatives and test them
• (nice to have) figure out how to return coordinate metadata (maybe signal should return the tuple (coords, values)? Or a separate (data) class with properties coords and values). Note that the coordinates are not needed anywhere in the code, they would just be there as a convenience for the user, in case they want to construct the derivatives themselves, or if they want to easily plot the derivatives without re-computing the coordinates.
• (nice to have) implement adaptive step size so the user doesn't have to guess whether the derivative computed has actually converged (and hasn't experienced underflow)
• implement interfaces to various third-party software:
  • Add interface to CLASS #44
  • CAMB
  • PyCosmo
• (nice to have) make a metaclass that can be used to streamline the creation of interfaces to third-party software
• (nice to have) given an interface to some third-party software, make it possible to specify which parameters we can be varied, so the user does not have to guess ("do I use omega_m? Omega_m? Omega_matter?")
• (nice to have) implement computation of derivatives using the complex step approximation
• (nice to have) develop interface for software that supports automatic differentiation #42