Simulation refactor

[brenjohn]

Summary

The design using global and sub communicators to iterate over open and sliced bonds respectively has been replaced with a design outlined below:

• A simulation is considered as a set of contraction jobs parameterised by bitstring-slice pairs.
• Contraction jobs are divided amongst MPI processes evenly
• Each process spawns a local julia cluster using the Distributed package consisting of a master process and 1 worker process for each gpu device available to the cluster. The master process starts a job queue from which worker processes take contraction jobs. Sliced amplitudes are collected on the master process via an ampltude queue.
• All results are collected and written to memory on the root MPI process at the end of a simulation.

Rationale

Changes allow multiple GPU devices on a single node to be used and simplifies work distribution.

Implementation Details

Additional notes

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Check before merging:

• All discussions are resolved
• New code is covered by appropriate tests
• Tests are passing locally and on CI
• The documentation is consistent with changes
• Any code that was copied from other sources has the paper/url in a comment and is compatible with the MIT licence
• Notebooks/examples not covered by unittests have been tested and updated as required
• The feature branch is up-to-date with the master branch (rebase if behind)
• Incremented the version string in Project.toml file

[codecov-commenter]

Codecov Report

Merging #57 (d416892) into master (d3d0fa0) will increase coverage by 5.32%.
The diff coverage is 81.47%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   75.79%   81.12%   +5.32%     
==========================================
  Files          14       14              
  Lines         537      625      +88     
==========================================
+ Hits          407      507     +100     
+ Misses        130      118      -12     

Impacted Files	Coverage Δ
src/compute_graph/tree.jl	90.62% <ø> (ø)
src/compute_graph/tree_stats.jl	2.27% <0.00%> (-2.73%)	⬇️
src/sysimage/precompile.jl	0.00% <0.00%> (ø)
src/utils.jl	76.66% <76.66%> (ø)
src/simulation_contexts/amplitude_channel.jl	84.00% <84.00%> (ø)
src/contexts/base.jl	85.57% <88.88%> (-11.54%)	⬇️
src/logger.jl	88.88% <91.52%> (+19.12%)	⬆️
src/simulation_contexts/simulation_contexts.jl	91.83% <91.83%> (ø)
src/compute_graph/dsl.jl	77.27% <100.00%> (+0.71%)	⬆️
src/simulation_contexts/amplitude_simulation.jl	100.00% <100.00%> (ø)
... and 2 more

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