Fix bug in ML forecasting if observables are not included
Fixing bug introduced in ae82c284dcc11714e39a29e11400ee79af8df02e
Add comment why use of old correlation matrix from previous draw is correct and revert change

Due to only using the diagonal of Sigma_e and the correlation matrix having ones on the diagonal, the diagonal entries of the covariance matrix are correctly built from recent draw. Later, when using the new draw for the correlations, only the correctly updated diagonal entries of Sigma_e are used.
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
-
Add unit test for correlated measurement error

Tests both ML and Bayesian estimation and test calibrated correlation of
those shocks
Clarify treatment of measurement error and correlations in manual
Merge commit '34fb9f1c62c63e7e4ab41656a7690167449e596f'
Merge pull request #329 from JohannesPfeifer/master

Fixing of potentially serious bugs in estimation