mrc-3825: Use dust approach to method provision on initialisation

[richfitz]

Because dust supports both gpu and cpu models there's a bit of a faff with setting up pointers to the actual methods. Mode has never needed that but will once we share a template. There's a corresponding tweak in the .cpp itself to allow the template substitution to pick the target (here ode) and the container (here dust_ode)

With this, the dust.R.template and mode.R.template should be basically identical aside from swapping mode for dust. The mode.cpp file contains the same contents as the associated files in dust (it's split across a few files).

There's a couple of other small tweaks here:

• a new param method (and corresponding empty bit in the template); this will become clearer once we merge with dust as there's a dance there to get nice parameter information (what does the model need in order to be created) from it
• tweaks to the state() method to move the logic from the R template into the C++, matching dust
• a new gpu_info method which I should have added in Add dust-style capabilities testing #41 probably, but does very little

[codecov]

Codecov Report

Base: 95.89% // Head: 96.80% // Increases project coverage by +0.91% 🎉

Coverage data is based on head (498e0df) compared to base (cfc1ba5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   95.89%   96.80%   +0.91%     
==========================================
  Files           3        3              
  Lines          73       94      +21     
==========================================
+ Hits           70       91      +21     
  Misses          3        3              

Impacted Files	Coverage Δ
R/interface.R	97.14% <100.00%> (+1.22%)	⬆️

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