pgr_breadthFirstSearch GSOC-2019 week 4

[vicennial]

• Added a negative value check on the 'max_depth' input parameter of the pgr_breadthFirstSearch function. An error is thrown to the user instead of displaying an empty result.
• Modified the implemented algorithm to return the traversal starting at depth zero instead of depth one. This was done to avoid empty output when the input graph contains only a single vertex which is also the source vertex in the function call.
• Modified the implemented algorithm to return an empty table when the source vertex is not present in the input graph.
• Added function input/output parameter type-check pgTap tests.
• Added algorithm edge-cases pgTap tests.
• Added additional inner-query pgTap tests.
• Added more example queries and their results in the function's documentation page.