Use approximate sparsity detection by default

[avik-pal]

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the ScioML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

Additional context

If Symbolics.jl is not loaded, it uses ForwardDiff for approximate sparsity detection. I am displaying a warning because this can lead to failures due to incorrect sparsity pattern. But overall this is very promising at least for the brusselator example. Approximate Detection + Solving takes the similar time as Symbolics Solving (symbolic detection being extremely slow)

[codecov[bot]]

Codecov Report

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

Comparison is base (d7ef4af) 80.48% compared to head (893d084) 89.37%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #318 +/- ## ========================================== + Coverage 80.48% 89.37% +8.89% ========================================== Files 23 24 +1 Lines 1942 1949 +7 ========================================== + Hits 1563 1742 +179 + Misses 379 207 -172 ```

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