Generate code for models on gpu

[richfitz]

Uses the support introduced in https://github.com/mrc-ide/dust/pull/164 which is incomplete. This PR lets us generate fairly complicated update functions (theoretically that of sircovid?) which will make further development of the gpu code much easier.

Changes are relatively minor here:

• light rewriting of the equations involved in the update as we've moved a few things in internal
• some changes to the sum functions - note explicit specification of the return type now as the container type and value type are now unrelated
• additional argument to the main odin_dust function to turn on this support

It's not enabled by default, and not enabled at all in packages; that will give us freedom to tinker with the toolchain in dust

[codecov[bot]]

Codecov Report

Merging #64 (2a49a20) into master (c596ba0) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #64   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          693       770   +77     
=========================================
+ Hits           693       770   +77     

Impacted Files	Coverage Δ
R/generate_dust.R	100.00% <100.00%> (ø)
R/generate_dust_equation.R	100.00% <100.00%> (ø)
R/generate_dust_sexp.R	100.00% <100.00%> (ø)
R/odin_dust.R	100.00% <100.00%> (ø)
R/utils.R	100.00% <100.00%> (ø)

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