Restore error message for unparametrized Normal/MvNormal/Gamma

[bvdmitri]

Fixes #360

This pull request includes changes to the GraphPPL library interface to ensure that certain distributions cannot be constructed without keyword arguments, and corresponding tests to verify this behavior.

Changes to GraphPPL library:

• src/model/graphppl.jl: Modified the default_parametrization method for Normal, MvNormal, and Gamma to include GraphPPL.Atomic and an additional rhs parameter, ensuring these distributions cannot be constructed without keyword arguments. [1] [2]

Test additions:

• test/models/aliases/aliases_gamma_tests.jl: Added a test to verify that Gamma cannot be used as a node without keyword arguments.
• test/models/aliases/aliases_normal_tests.jl: Added tests to verify that Normal and MvNormal cannot be used as nodes without keyword arguments.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.19%. Comparing base (d4a202e) to head (1c0e49b). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #364 +/- ## ========================================== + Coverage 84.99% 85.19% +0.19% ========================================== Files 20 20 Lines 1526 1526 ========================================== + Hits 1297 1300 +3 + Misses 229 226 -3 ```

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