Make spike count plot optional

[bauerfe]

Checklist before requesting a review

• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)
• I have performed a self-review of my code
• Will this be part of a product update? If yes, please write one line about this on the CHANGELOG.md

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Improves DynapcnnVisualizer class by making spike count plot optional. Addresses Dynapcnn Visualizer: Enable excluding spike count plot #238 .

• What is the current behavior? (You can also link to an open issue here)
  GUI of DynapcnnVisualizer always shows spike count plot.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No.

[ssinhaleite]

Hi @bauerfe ,

I've reviewed your changes, and they look good.
I am merging it, but just FYI, I am adding two new issues that I caught while reviewing this code (but that are not directly related to this feature).

Thanks!

[ssinhaleite]

all set!