Hyperlink documentation to source code

[chaozg]

General description of task

Turn on link to source code in Sphinx

Definition of Done (Feature/change)

• turn on sphinx.ext.viewcode in configing Sphinx

Definition of Done (Mandatory)

• Documentation added (docstrings on all public methods/classes)
• Unit tests added/updated (and passing!)
• Code reviewed and approved by 2 CUQI-maintainers
• Online documentation is rendered correctly (esp. math)

Definition of Done (Optional)

• HowTo/tutorial added

[jakobsj]

@chaozg DoD approved.