Add GPU support

[richfitz]

This PR activates new GPU support in odin.dust.

Changes here are fairly minor:

• split generating gpu-like code from compiling with NVCC
• annotate sum functions with HOSTDEVICE
• annotate device update function with DEVICE
• special treatment of %%, min and max, as found empirically with sircovid

I've also added the sirs example used to develop dustgpu, but we might swap that out for something a bit easier to push around at some point

Requires https://github.com/mrc-ide/dust/pull/187 to be merged, and then the DESCRIPTION here updated to remove the branch pin

The codecov error is it complaining about a file that no longer exists :man_shrugging:

Fixes #66

[codecov[bot]]

Codecov Report

Merging #69 (8f1edd5) into master (6f7c41e) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #69   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         7    +1     
  Lines          838       856   +18     
=========================================
+ Hits           838       856   +18     

Impacted Files	Coverage Δ
R/common.R	100.00% <100.00%> (ø)
R/generate_dust.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)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 6f7c41e...8f1edd5. Read the comment docs.

[richfitz]

Are there any other odin functions other than these sums that might need a HOSTDEVICE?

Good question - I just checked and all the other functions are used just for the interface layer (so definitely just host based). If we add something like the fast round or accurate mod that'd need doing though