Structure of the config file

A dictionary containing:
cosmology:
- an astropy cosmology object
- a list of parameters to vary, each element of which is a dictionary with the fields: name, value, lower, upper

mean systematics:
- a name of a function which returns a mean covariance
- a list of parameters to vary, same as above

systematics covariance:
- one name for a function which returns a covariance matrix substructure (identity, block diagonal, band diagonal, fully correlated, etc.)
- a list of parameters for each function, same as above

structure of the data
- a vector of redshifts at which to make the measurements
- a string that tells us which measurement is being made.