Use Sparse instead of Dense for Connections

[SveaMeyer13]

** Tests are failing since we need to merge corresponding changes in Lava first**
Objective of pull request: Use Sparse connectivity in the connect function by default. Allow the User to specify a different connectivity.

Pull request checklist

Your PR fulfills the following requirements:

• Issue created that explains the change and why it's needed
• Tests are part of the PR (for bug fixes / features)
• Docs reviewed and added / updated if needed (for bug fixes / features)
• PR conforms to Coding Conventions
• PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• Lint (pyb) passes locally
• Build tests (pyb -E unit) or (python -m unittest) passes locally

Pull request type

Please check your PR type:

• Bugfix
• [x ] Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation changes
• Other (please describe):

What is the current behavior?

Connections are Dense by default, they cannot be changed by the user.

What is the new behavior?

Connections are Sparse by default, they can be changed by the user.

Does this introduce a breaking change?

• Yes
• [x ] No

[mathisrichter]

Some minor changes - looks good overall.

[gkarray]

Looks great!

[mathisrichter]

Please check whether you can reduce the code duplication that Codacy is pointing out.