Add a notebook demonstrating how to evolve objects

[engintoklu]

This pull request adds an example notebook which demonstrates how one can declare and solve an optimization problem that has its dtype set as object, and therefore has a custom solution structure.

While working on the example notebook, one bug, and one genetic algorithm-related shortcoming have been identified and fixed.

The bug was preventing one from instantiating a new ObjectArray with the help of evotorch.tools.as_tensor(..., dtype=object). This bug is fixed by this pull request.

The shortcoming was as follows: when the user defined a custom mutation operator using a regular function, GeneticAlgorithm assumed that the regular function would return a new batch of solutions whose length is equal to the number of solutions it received. This was an unnecessary restriction, and therefore, gets fixed with this pull request.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 76.73%. Comparing base (5c58566) to head (b74ca43). Report is 1 commits behind head on master.

:exclamation: Current head b74ca43 differs from pull request most recent head d600411

Please upload reports for the commit d600411 to get more accurate results.

Files	Patch %	Lines
src/evotorch/algorithms/ga.py	76.92%	3 Missing :warning:
src/evotorch/tools/misc.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #102 +/- ## ========================================== - Coverage 76.74% 76.73% -0.01% ========================================== Files 49 49 Lines 7509 7519 +10 ========================================== + Hits 5763 5770 +7 - Misses 1746 1749 +3 ```

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