python-experimental adds DecimalSchema

[spacether]

python-experimental adds DecimalSchema (type: string format: number)

• DecimalSchema allows user to send number amounts as strings to preserve the desired sig figs.
• I also added isDecimal to CodegenModel; this was needed to get the feature working
• Decimal changed to decimal.Decimal in case someone creates a component named Decimal
• An import path in model tests was fixed for python-experimental
• The __new__ methods for models now returns ModelName so pycharm type hints will show instance properties on instances created with inst = ModelName({})

Note: for DecimalSchemas like these, one could also define string validations like a regex that requires a specific number of sig figs.

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