Feature/getindex for trajectory

[adolgert]

Made an enabled() that is efficient for memory.
Lots and lots of little bug fixes.
Standardized across samplers.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 62.50000% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.89%. Comparing base (6686b36) to head (fa1faa9).

Files with missing lines	Patch %	Lines
src/sample/interface.jl	0.00%	12 Missing ⚠️
src/context.jl	0.00%	7 Missing ⚠️
src/trace/trajectory.jl	14.28%	6 Missing ⚠️
src/sample/neverdist.jl	20.00%	4 Missing ⚠️
src/prefixsearch/binarytreeprefixsearch.jl	95.08%	3 Missing ⚠️
src/prefixsearch/cumsumprefixsearch.jl	70.00%	3 Missing ⚠️
src/sample/multiple_direct.jl	50.00%	3 Missing ⚠️
src/sample/sampler.jl	0.00%	3 Missing ⚠️
src/sample/direct.jl	0.00%	2 Missing ⚠️
src/sample/firstreaction.jl	0.00%	2 Missing ⚠️
... and 5 more
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Additional details and impacted files

@@                  Coverage Diff                   @@
##           dev/sample-context      #89      +/-   ##
======================================================
- Coverage               68.58%   67.89%   -0.70%     
======================================================
  Files                      21       21              
  Lines                     955      978      +23     
======================================================
+ Hits                      655      664       +9     
- Misses                    300      314      +14     

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