Collect observational constraints

[sambit-giri]

We have a module collecting constraints on the reionization history (tools21cm.compare_constraints.ReionizationHistory). We should expand this library with more of these constraints and other quantities, such as ultraviolet luminosity functions and Lyman-α continuum escape fractions.

[sambit-giri]

@micbia In this commit, I included the Lyman-alpha escape fraction data that you collected. It can be used through tools21cm.compare_constraints.EscapeFractionLymanAlpha.