Sample simultaneously

[richfitz]

Merge after #34, contains those commits

This PR adds support for simultaneously sampling from multiple chains at once. This is triggered by having pars be a matrix rather than a vector when passed into a sampler/model. Models will need to be able to support this and we don't assume that they do; the property allow_multiple_parameters can be used to signal this.

This functionality has two purposes:

• to support a generic implementation of parallel tempering, where we will embed a sampler (e.g, random walk, hmc or adaptive walk) within a parallel tempering scheme, requiring us to have each sampler step multiple chains at a single time
• to allow more efficient sampling from simple models (e.g., those that come from the DSL would be easy to generate to support this).

There are lots of changes here, but I think it does all need to come though at once or it loses context. Let's chat through the ideas this week.

Things to note:

• I need to tidy up how details is collected at the end of a run; I've not done this even though it's only a few lines
• I do not like allow_multiple_parameters as a name, but can't think of a better one
• support for stochastic models is a little hard, and also causes some issues with the wrapper we currently have for dust models; I've punted on that from now and disallow it

[codecov]

Codecov Report

Attention: Patch coverage is 98.87640% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 99.89%. Comparing base (b588969) to head (77cb30c).
Report is 74 commits behind head on main.

❗ Current head 77cb30c differs from pull request most recent head f83d225. Consider uploading reports for the commit f83d225 to get more accurate results

Files	Patch %	Lines
R/sampler-hmc.R	96.29%	2 Missing ⚠️
R/sample.R	97.22%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   99.44%   99.89%   +0.44%     
==========================================
  Files          45       53       +8     
  Lines        2163     2859     +696     
==========================================
+ Hits         2151     2856     +705     
+ Misses         12        3       -9     

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