Add IF2 algorithm

[johnlees]

See:

• https://kingaa.github.io/pomp/vignettes/mif2.html
• https://kingaa.github.io/sbied/mif/if2_settings.html
• https://kingaa.github.io/sbied/mif/notes.pdf

TODO:

• Make an IF2 control class
• Add full set of tests for IF2 parameter class
• Add full set of tests for IF2 class
• Add full set of tests for IF2 control class
• Docstrings (including examples)

Fixes #123

[codecov]

Codecov Report

Merging #133 (d39737f) into master (a252242) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #133    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           21        24     +3     
  Lines         2140      2291   +151     
==========================================
+ Hits          2140      2291   +151     

Impacted Files	Coverage Δ
R/smc2_parameters.R	100.00% <ø> (ø)
R/if2.R	100.00% <100.00%> (ø)
R/if2_control.R	100.00% <100.00%> (ø)
R/if2_parameters.R	100.00% <100.00%> (ø)

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[richfitz]

Looks good so far - as you say it's not a massive piece of implementation. Or perhaps we're both broken by the cuda stuff.

In addition to the comments, could you add a vignette, no matter how brief, showing using this on either the sir or volatility model?

[richfitz]

This is quite a different interface to that presented by pmcmc and I'd like to harmonise if possible. There, we have the functional interface where calling the function pmcmc returns a plain list of class mcstate_pmcmc which has pars, likelihoods etc. On the way it uses a semi-exported object pmcmc_state but that's an implementation detail.

So here, the if2 function should use the object much as you have it, but return all the requested information as a list. Unless there's some reason that's not possible, in which case please let me know.