tests: improving tests for tracing functions
utilities|tests: tweaking assertion methods and adding tests for these
quadrature nodes: introduced ABC; adjusted/extended/fixed tests
nodes: made number nodes an optional kwarg
weight_functions: made interface an Abstract Base Class
PolynomialWeightFunction: adjustments/tweaks/fixes
tests: for PolynomialWeightFunction
quadrature: some mode and class renaming

mainly to shorten names and made naming consistent/intuitive
quadrature: tweaks/cleanup/improvements
tests: quadrature tests completed
Merge branch 'feature/abstracting-quadrature' into feature/the-big-cleanup
ABCs: remove unused Comparable ABC
adjustments to recent renaming
tests: add tests on copyability of data structures
Merge branch 'feature/utilities-tests' into feature/the-big-cleanup