Support min/max values

[Cantido]

The schema for QuantitativeValue has fields for minValue and maxValue. Since the telnet responses sometimes contain an error estimate, like +-0.01, we can calculate this min and max as well. Currently, these error estimations are totally ignored.

Note that some measurements have decimal-place-dependent error measurements, like 1.123+-4, which really means 1.123+-0.004.