Cosmological Constraints on sub-horizon scales modified gravity theories
  with MGCLASS II
  In this paper we introduce a new public Einstein-Boltzmann solver,
\texttt{MGCLASS II}, built as a modification to the publicly available
\texttt{CLASS} code, that allows to obtain cosmological observables for
Modified Gravity theories. It implements several commonly used
parameterizations of deviations from General Relativity, computing their impact
on the growth of structure as well as on the background evolution of the
Universe, together with a subset of available alternative theories, still not
completely ruled out by observations. \texttt{MGCLASS II} is built in such a
way to be compatible with parameter estimation codes such as
\texttt{MontePython} and \texttt{Cobaya}. We exploit this possibility to
constrain the parameterizations used by the Planck collaboration, in order to
validate the predictions of this new code, and a newly implemented
parameterization (z\_flex) which has different features. For the former we find
good agreement with the results existing in the literature, while we present
original constraints on the parameters of the latter, finding no significant
deviation from the standard cosmological model, $\Lambda$CDM.
