Rationalise names used for gpu things

[richfitz]

This PR rationalises names used for gpu things:

• most uses of device/cuda become gpu, especially when facing the user; this affects files, namespace and functions!
• exceptions include "device_id", actual device storage (an implementation detail really) and things to do with the compilation toolchain
• At the same time (to minimise the number of disruptive changes) the interface namespace becomes r, which will make Add simple python bindings to the dust class #271 much easier
• New check to make sure that header guards are correct

I've tried to avoid folding any features in here, but couldn't help adding a fix for #301 because I was interacting with code that is very nearby.

Fixes #317
Fixes #304
Fixes #301
Fixes #298

[codecov]

Codecov Report

Merging #321 (92fc6a4) into master (c6d40d4) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 92fc6a4 differs from pull request most recent head db81b79. Consider uploading reports for the commit db81b79 to get more accurate results

@@            Coverage Diff            @@
##            master      #321   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           57        57           
  Lines         3258      3258           
=========================================
  Hits          3258      3258           

Impacted Files	Coverage Δ
R/interface.R	100.00% <ø> (ø)
inst/include/dust/gpu/cuda.hpp	100.00% <ø> (ø)
inst/include/dust/gpu/device_state.hpp	100.00% <ø> (ø)
inst/include/dust/gpu/gpu_info.hpp	100.00% <ø> (ø)
inst/include/dust/r/random.hpp	100.00% <ø> (ø)
inst/include/dust/r/utils.hpp	100.00% <ø> (ø)
inst/include/dust/random/normal.hpp	100.00% <ø> (ø)
R/compile.R	100.00% <100.00%> (ø)
R/cuda.R	100.00% <100.00%> (ø)
R/metadata.R	100.00% <100.00%> (ø)
... and 30 more

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[johnlees]

has something gone wrong with the formatting here?