compiler: Add Weights.value utility

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 27.27273% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 86.66%. Comparing base (edd6a2e) to head (3b00540).

Files	Patch %	Lines
devito/finite_differences/differentiable.py	27.27%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2322 +/- ## ========================================== - Coverage 86.67% 86.66% -0.02% ========================================== Files 229 229 Lines 43153 43164 +11 Branches 8002 8004 +2 ========================================== + Hits 37402 37407 +5 - Misses 5054 5060 +6 Partials 697 697 ```

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