Dense floating-point vector attributes

[averywang21]

Pull request contains:

• dense floating-point vector attribute support with public methods analogous to the existing int vector attribute methods.
• iterators for floating-point vector attributes (an iterator for each element for a particular attribute)
• improved tests for int vector attribute methods, new tests for floating vector attribute methods
• added tests for int and float vector attribute delete functions
• fixed bugs in original int vector attribute edge methods - incorrect use of edge ID vs. node ID.
• implementation of sparse floating-point vectors and their iterators.
• tests: iterators for int vector attributes (originally lacking) and floating-point vector attributes, both sparse and dense representations.

[averywang21]

Finalized iterator tests.

[roks]

Thank you very much for your contributions!