Nodemodel

What's Changed
- Allow using callable objects as nodes
- load_nodes function will load also objects defined as items of dictionaries

What's Changed
* add doc strings to all functions and classes
* add readthedocs

What's Changed
* Add more docstrings/unit tests
* General refactoring
* New feature: forced nodes values can be another nodes


**Full Changelog**: https://github.com/krzysztof-pankow/nodemodel/compare/v0.1.2...v0.1.3

What's Changed
* Check acyclicity in the graph
* Include isolated nodes in the graph
* Add **kwargs in compute method - arguments to be temporarily added to the input dictionary


**Full Changelog**: https://github.com/krzysztof-pankow/nodemodel/compare/v0.1.1...v0.1.2

What's Changed
* correct pyproject file by krzysztof-pankow in https://github.com/krzysztof-pankow/nodemodel/pull/4