Extend available methods

[nk027]

At the moment only plot() and print() exist for the bvar object.

Important standard methods are:

• fitted(): Probably better to calculate ex-post.
• predict(): This is currently done in the main chain, as it shouldn't necessitate too much additional effort. Allowing to calculate this ex-post should not be an issue, however.
• summary(): This really needs to be readded.
• residuals(): See fitted()

Further potential methods are:

• coef(): Should be useful and rather straightforward. Optionally consider the vars implementation of Acoef() and Bcoef().
• vcov(): Should also be straightforward.
• logLik(): Might as well.
• irf(): See predict().
• fevd(): Nice shorthand.
• stability(): Additional dependency, but might make sense.
• Phi() & Psi(): Would address facilitating switches from and to vars.

• The user interface of BVAR has bvar as the main fitting function
  returning an object that only has a plot() and a print() method. First,
  other standard methods like fitted(), predict(), residuals(), and
  summary() should be added. Second, methods that are typically available
  for frequentist models like coef(), vcov(), logLik() etc. might also make
  sense in this Bayesian setting. Third, the "vars" package defines generics
  and methods for a lot of VAR-specific functionality, e.g., Phi(), Psi(),
  fevd(), irf(), stability(). As already mentioned in the previous point it
  would be very natural to provide methods to these generics as well.

[nk027]

predict, irf, fitted, residuals and fevd have been implemented as of 92286d2.

This includes methods for different objects that make sense in this context. Beside calculating stuff ex-post the main functionality is calculation of quantiles and improved printing, etc.

[nk027]

• Commit 0dfcce2 contains some preliminary work on logLik().
• summary() should probably collect the information and outputs of other methods, so it might make sense to implement it last.
• stability() is not a priority right now.
• Phi() and Psi() methods seem to not offer much - a method to retrieve the companion matrix probably makes more sense.

[nk027]

I'll add summary() tomorrow.
Besides that #27 could use some work.

(Remember to mention density != HPDI)

[nk027]

Thanks @oDNAudio for adding the summary method.

I've split it up to return the coefficients and feed that to a print-method.

Medium term it might make sense to create more selected outputs and allow retrieving some HPDIs. But I think that can be done in another issue.

Closing this, as all must-add methods have been added.