Implement lazy node label for easier datavars construction

[bvdmitri]

This PR adds a possibility to create datavar automatically on demand without explicitly specifying their dimensions in advance. This is done by implementing LazyIndex which materializes node labels upon usage.

Additionally this PR implements a new syntax in the model specification with the new operator which will force create a new node label even if it wasn't defined before.

[bvdmitri]

This PR also adds an error message for the following case

@model function simple_model_with_wrong_indexing()
    x ~ MvNormal([ 0.0, 0.0 ], [ 1.0 0.0; 0.0 1.0 ])
    y ~ Beta(x[1], x[2])
end

Indexing a single node label `x` with an index `[1]` is not allowed.

[codecov]

Codecov Report

Attention: Patch coverage is 86.11111% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 91.06%. Comparing base (5028424) to head (3203330).
Report is 9 commits behind head on dev-4.0.0.

Files	Patch %	Lines
src/graph_engine.jl	85.83%	17 Missing ⚠️
src/model_macro.jl	86.95%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           dev-4.0.0     #155      +/-   ##
=============================================
+ Coverage      90.50%   91.06%   +0.55%     
=============================================
  Files             14       14              
  Lines           1612     1724     +112     
=============================================
+ Hits            1459     1570     +111     
- Misses           153      154       +1     

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