Optimising parametrisation in config file

[philipp128]

Description

Enable to have an easier option for the parametrisation in the config file.

Current Problem

Let's look at the case that we want to sample galaxies from the Schechter function. An example config file is mccl_galaxies.yml in skypy/examples.
The model of phi_star is given as an exponential function b*exp(a*z) where b and a are the model parameters.
At the moment we are using Astropy's astropy.modeling.models.Exponential1D function to parametrise this model. However, this requires the input parameter tau which corresponds to 1/a.

As usually the parameter a is given in the literature the user has to calculate the parameter tau by hand and add it to the config file. A more user-friendly way might be good for the future.

Suggestion

• Option to include lambda functions in the config file to construct your own models
• Enable calculations in the config file such that it is possible to write tau as 1/a