Reorganize into multi-package monorepo

[GordStephen]

This splits out functionality into separate packages, PRASCore, PRASFiles, and PRASCapacityCredits, and sets the stage for moving additional packages into the repo as well (e.g. PRASAnalytics). PRAS is retained as a meta-package that automatically loads the other packages. Tests run on each package individually but coverage reporting is done repository-wide (https://app.codecov.io/gh/NREL/PRAS/tree/main).

This simultaneously removes support for the Convolution simulation specification, and more generally removes the infrastructure for using simulation specifications other than SequentialMonteCarlo. This simplifies the existing codebase and also provides a more gradual transition path for users towards the Rust rewrite, which will only support chronological Monte Carlo simulation.

Although no longer necessary, for backwards-compatibility the (system, simspec, resultspec) triple API is retained for assess calls, although assess(sys, Convolution(), resultspecs...) will no longer work.

Given the above breaking changes, this also updates each package version to 0.7.0, which can be the first version we submit to the General registry.

I also pulled in @josephmckinsey's docstring commit and augmented it with dccf8d7.

I'm thinking we won't rebase this PR onto master, but rather just switch the repo default branch over to this one (main) when we're ready. Anyone working with a PRAS#master package install will stay on v0.6.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.97737% with 85 lines in your changes missing coverage. Please review.

Project coverage is 83.05%. Comparing base (c56a257) to head (bc3c467).

Files with missing lines	Patch %	Lines
PRASCore.jl/src/Results/Results.jl	51.16%	21 Missing ⚠️
PRASCore.jl/src/Simulations/recording.jl	88.27%	19 Missing ⚠️
PRASCore.jl/src/Results/GeneratorStorageEnergy.jl	80.76%	5 Missing ⚠️
PRASCore.jl/src/Results/GeneratorAvailability.jl	77.77%	4 Missing ⚠️
...ore.jl/src/Results/GeneratorStorageAvailability.jl	77.77%	4 Missing ⚠️
...re.jl/src/Results/GeneratorStorageEnergySamples.jl	80.95%	4 Missing ⚠️
PRASCore.jl/src/Results/LineAvailability.jl	77.77%	4 Missing ⚠️
PRASCore.jl/src/Results/StorageAvailability.jl	77.77%	4 Missing ⚠️
PRASCore.jl/src/Results/StorageEnergySamples.jl	80.95%	4 Missing ⚠️
PRASCore.jl/src/Results/FlowSamples.jl	87.50%	3 Missing ⚠️
... and 7 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   90.38%   83.05%   -7.33%     
==========================================
  Files          39       40       +1     
  Lines        1955     1794     -161     
==========================================
- Hits         1767     1490     -277     
- Misses        188      304     +116     

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[sriharisundar]

Can change some @asserts in doc strings to maybe show sample outputs, but let's track that as an issue and take care of it later?

[GordStephen]

Thanks all! Closing since this is now the default branch for the repo.