Fix various Kalman filter bugs

• Fix bug in dsge_likelihood.m where diagonal of measurement error covariance matrix was not correctly read out, leading to wrong results when using the univariate Kalman filter with measurement error
• Various bugfixes for univariate smoother when (correlated) measurement error is present
• Bugfix for missing_observations_kalman_filter_d.m where constant of log(2*pi) was forgotten in last step
• Documents Kalman routines
• Factorizes and improves unit tests
• Filters out infinite likelihoods with proper error code
• Fixes variable count bug in display_problematic_vars_Jacobian.m that led to crashes in debug-mode

Closes #1231 (closed)

Closes #1234 (closed)