Add Decimal data type

Add a new basic data type(BDT) to SMPL called "Decimal" that uses the Java [BigDecimal](https://docs.oracle.com/javase/10/docs/api/java/math/BigDecimal.html) Class BTS for the option of using precise real number values.

proposed syntax:
> `#d0.4 // returns 0.4

- could provide more available ops to perform on Decimal than just the regular numeric ops real has.
- should be able to substitute Real for Decimal and convert Real to Decimal

Start by locating the existing Real basic data type and the interpreter's handling of numeric operations and conversions. Review the proposed `#d0.4` syntax and Java BigDecimal reference, then clarify the supported operations, Real substitution, and Real-to-Decimal conversion. Done means Decimal is parsed and behaves as specified throughout those cases.

Written by the indexing model from the issue text.