Add rounding of extremely small threshold values

This MR addresses and closes #93 (closed) which noted that extremely small threshold values (e.g., 1e-17) can be returned.

Values below three decimal places (which is smaller than the minimum precision that NWIS records) are rounded to zero now. No other changes to rounding or precision are implemented in this MR. As noted in #76 , additional rules on rounding and precision should be considered but are waiting on further guidance