Refactor TensorMethod

[drhagen]

• Rename PureTensorMethod to TensorMethod
• Base TensorMethod on Problem
• Make tensor method parameters keyword only
• Make most attributes of TensorMethod private
• Make all AST objects fully immutable so that they are hashable

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (1a83d89) 89.02% compared to head (267e1a0) 89.08%.

Files	Patch %	Lines
src/tensora/problem.py	66.66%	1 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #44 +/- ## ========================================== + Coverage 89.02% 89.08% +0.06% ========================================== Files 54 54 Lines 2951 2960 +9 Branches 526 527 +1 ========================================== + Hits 2627 2637 +10 + Misses 248 247 -1 Partials 76 76 ```

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