Fix CIPWnorm output discrepancy

[bomtuckle]

Description

Made changes to CIPWnorm function to remove errors in the output that were occurring due to a problem with the correction of mineral molecular masses.

Related Issue

CIPWnorm feature #53

Motivation and Context

Make the output fully comparable with SINCLAS/IgRoCs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 1927909317

• 87 of 88 (98.86%) changed or added relevant lines in 1 file are covered.
• 21 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 89.242%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyrolite/mineral/normative.py	87	88	98.86%

Files with Coverage Reduction	New Missed Lines	%
pyrolite/mineral/normative.py	21	91.43%

Totals
Change from base Build 1793156490:	-0.2%
Covered Lines:	5790
Relevant Lines:	6488

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💛 - Coveralls

[morganjwilliams]

I've had another look at CIPW in light of this working as expected and found a bug or two in the molecular masses (principally that using existing parsing,"2MgO SiO2" != "(MgO)2 SiO2"); I'll pull this in and address anything residual I can see.