[MRG] ENH: Add ability to define cell grid dimensions

[ntolley]

As discussed in #704, we've realized that it is necessary to have an explicit API fro defining the size of the network object. Previously this was done in a hacky way by directly editing the params object. The proposed API here is

net = jones_2009_model(mesh_shape=(10, 10))

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (1216038) 91.34% compared to head (1f25b11) 91.36%. Report is 46 commits behind head on master.

:exclamation: Current head 1f25b11 differs from pull request most recent head 3a069cd. Consider uploading reports for the commit 3a069cd to get more accurate results

Files	Patch %	Lines
hnn_core/gui/_viz_manager.py	83.87%	5 Missing :warning:
hnn_core/gui/gui.py	92.30%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #705 +/- ## ========================================== + Coverage 91.34% 91.36% +0.02% ========================================== Files 25 25 Lines 4599 4611 +12 ========================================== + Hits 4201 4213 +12 Misses 398 398 ```

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[ntolley]

@gtdang finally got the tests passing! just updated whats_new.rst, merge button is yours once the checks are green

[gtdang]

@gtdang finally got the tests passing! just updated whats_new.rst, merge button is yours once the checks are green

Awesome! Great job!

[jasmainak]

Thanks @gtdang and @ntolley for taking this through!