Add bandpass filter, one-sided HP filter, and simulated variance decomposition
- Closes #1011 (closed)
- Fixes UnivariateSpectralDensity.m for filtered variables
- Adds better documentation
- Closes #1012 (closed)
TBD: Interface for spectral density as discussed in https://github.com/DynareTeam/dynare/issues/254 (unit test then needs to be adjusted)