Dev symbolic

[tobin-ford]

Describe your changes

• symbolic equation solver for simple models.
• notebook tutorial Custom-Functions-Nopython.ipynb

Issue ticket number and link

49

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] Code changes are covered by tests.
• [x] Code changes have been evaluated for compatibility/integration with Scenario analysis (for future PRs)
• [x] New functions added to init.py
• [x] API.rst is up to date, along with other sphinx docs pages
• [x] What's new changelog has been updated in the docs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.55319% with 7 lines in your changes missing coverage. Please review.

Project coverage is 64.70%. Comparing base (1b70fbc) to head (5cd650c).

Files	Patch %	Lines
pvdeg/symbolic.py	84.78%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #104 +/- ## =============================================== + Coverage 63.86% 64.70% +0.83% =============================================== Files 32 34 +2 Lines 3119 3213 +94 =============================================== + Hits 1992 2079 +87 - Misses 1127 1134 +7 ```

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