Change code to only use `Decimal` internally

[austinmcconnell]

The current code has liberal use of Decimal format as both internal and external functions. This means that users have to create Decimal objects for inputs and handle Decimal objects as outputs. It also makes the string representation of objects difficult to read (e.g. Installments). This is less than ideal.

Change the code to abstract away the use of Decimal objects so they are only used internally / behind-the-scenes. User-facing inputs and outputs should use nicer formats (potentially just floats or ints that are converted and stored in the backend).