Make two node example graph

[mscroggs]

• Refactored node and graph so that a graph can be more easily created using functions as in Helper class for creating graph #7.
• Added functions to estimate mean and standard deviation from a graph
• Added test that one and two node graphs containing normal distributions give the correct mean and stdev

[willGraham01]

Working example 🥳 🚀

Couple of small ruff-please-ers in the Graph class.

For the algorithms module, I'm not convinced as to why the methods in there can't be part of the Graph class itself, but I'm open to being convinced.

The test comments are mostly on a reduction in the number of cases. The two example tests have 16 & 32 test cases, respectively (which is not a bad thing, but some are likely redundant in terms of what they check).

[willGraham01]

Have put a few small suggestions in (if you agree with them, it should just be a case of "apply suggestion" and things won't break), otherwise looks good to go!