Ensure pdfs are reset and subtypes updated.

[daffidwilde]

Resetting:

• Previously, running an example twice gave different results
• This was because each pdf passed to the algorithm would develop subtypes in the first run.
• Now, each pdf family is reset at the start of the run so they have no subtypes. I'm hoping garbage collection catches this.

Updating subtypes:

• The available subtypes from which to generate columns is now updated for each pdf from those present in a generation's parents.

[coveralls]

Pull Request Test Coverage Report for Build 15

• 13 of 13 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 13:	0.0%
Covered Lines:	431
Relevant Lines:	431

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💛 - Coveralls