Add collection of mean, var and std for distributions

[theogf]

This adds direct accessors to the statistics of some given Measures

Let me know if this would be the wrong way to do this?

[github-actions]

Package name	latest	stable
Mitosis.jl
Soss.jl

[codecov]

Codecov Report

Patch coverage: 90.00% and project coverage change: +0.35 🎉

Comparison is base (a3b8f6d) 45.98% compared to head (4c9cff6) 46.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   45.98%   46.33%   +0.35%     
==========================================
  Files          43       43              
  Lines        1344     1364      +20     
==========================================
+ Hits          618      632      +14     
- Misses        726      732       +6     

Impacted Files	Coverage Δ
src/MeasureTheory.jl	50.00% <ø> (ø)
src/combinators/product.jl	10.71% <50.00%> (+3.02%)	⬆️
src/parameterized/normal.jl	52.08% <83.33%> (+4.46%)	⬆️
src/parameterized/mvnormal.jl	43.47% <100.00%> (+13.74%)	⬆️
src/parameterized/poisson.jl	70.00% <100.00%> (+5.29%)	⬆️

... and 1 file with indirect coverage changes

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[theogf]

Issues with the dependent packages seems to be linked to compatibilities issues.

[github-actions]

Package name	latest	stable
Mitosis.jl
Soss.jl

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.85%. Comparing base (84440f8) to head (8728097).

Files with missing lines	Patch %	Lines
src/combinators/product.jl	50.00%	1 Missing ⚠️
src/parameterized/normal.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   52.60%   52.85%   +0.24%     
==========================================
  Files          42       42              
  Lines        1399     1419      +20     
==========================================
+ Hits          736      750      +14     
- Misses        663      669       +6     

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[theogf]

That's a really weird bug:
https://github.com/JuliaMath/MeasureTheory.jl/actions/runs/7678848125/job/20929123594?pr=250#step=347:347

Although constructed with one value, the output is different

[theogf]

Looks like the issue is unrelated?