Fix SampleFromUniform

[mohamed82008]

This PR fixes the SampleFromUniform code. SampleFromUniform is only useful in initializing the HMC state. This means that:

1. A new value should always be sampled overwriting the existing values in VarInfo which may have come from SampleFromPrior, and
2. The "trans" flag should be set to true because we are not sampling in the support of the random variables' distributions.

This PR also fixes the uniform distribution's support as per @xukai92's suggestion.

[yebai]

@mohamed82008 Looks good to me, but seems some tests are broken due to these changes.

[mohamed82008]

Yes the broken tests test the old behavior. I think removing them should be ok.

[devmotion]

Can we finish this PR? I'm waiting for these fixes to adjust/correct the tests in #77 (if that PR is supposed to be merged).

[mohamed82008]

Sure, I will finish it tonight.

[devmotion]

Thanks!

[codecov]

Codecov Report

Merging #78 into master will decrease coverage by 0.54%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   77.46%   76.92%   -0.55%     
==========================================
  Files          13       13              
  Lines         821      845      +24     
==========================================
+ Hits          636      650      +14     
- Misses        185      195      +10     

Impacted Files	Coverage Δ
src/utils.jl	53.06% <50.00%> (ø)
src/context_implementations.jl	56.46% <54.54%> (+1.99%)	⬆️
src/varinfo.jl	83.90% <100.00%> (-0.53%)	⬇️
src/compiler.jl	87.82% <0.00%> (-0.21%)	⬇️
src/varname.jl	76.47% <0.00%> (ø)

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[mohamed82008]

Once tests pass, this can be merged. I will sleep now.

[devmotion]

I'm sure tests will fail on Julia master, at least that's what happened in all other PRs in the last days.