cumbof/hdlib: hdlib v0.1.15

hdlib v0.1.17

Add:

• Add the subtraction operator to the arithmetic module;
• Add __sub__ to space.Vector that makes use of arithmetic.subtraction to element-wise subtract two Vector objects;
• Add space.Vector.subtraction to element-wise subtract a vector from a Vector object inplace;
• Add graph.Graph to build vector-symbolic representations of directed and undirected, weighted and unweighted graphs;
• Extend test/test.py with new unit tests.