Fix several bugs related to estimated measurement errors

1. The first call to set_prior overwrote the first column of
estim_params_.var_endo storing the position of the variable with
measurement error in M_.endo_names with the position in
options_.var_obs. All subsequent calls to set_prior then lead to
crashes.
2. At the same time, for correlations of ME, the first column of
estim_params_.corrn still stored the position of the variable with
measurement error in M_.endo_names. But subsequent calls to it were done
as if it stored the position in options_.var_obs

I introduced two new variables in estim_params_ storing the respective
positions in var_obs so as to not necessitate changes in the
preprocessors.

3. For cases of calibrated measurement error correlations, the
covariance matrix was not updated.

4. Fixing a lot of smaller bugs related to measurement errors, including
some copy and paste errors
-
(cherry picked from commit 7518072e)