Add constants, expression maths, better testing and more

[chrisburr]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 27

• 151 of 160 (94.38%) changed or added relevant lines in 6 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+8.6%) to 85.119%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
formulate/expression.py	88	97	90.72%

Files with Coverage Reduction	New Missed Lines	%
formulate/parser.py	2	90.4%
formulate/expression.py	7	83.65%

Totals
Change from base Build 22:	8.6%
Covered Lines:	429
Relevant Lines:	504

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 47

• 212 of 230 (92.17%) changed or added relevant lines in 7 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+9.2%) to 85.714%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
formulate/parser.py	35	38	92.11%
formulate/logging.py	42	47	89.36%
formulate/expression.py	98	108	90.74%

Files with Coverage Reduction	New Missed Lines	%
formulate/expression.py	5	84.12%

Totals
Change from base Build 22:	9.2%
Covered Lines:	468
Relevant Lines:	546

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💛 - Coveralls