Stiffness/covariance functionality #3
Description
Based on willat343/mathbox#6 the covariance and stiffness from json functionality should be moved or improved (currently at https://github.com/willat343/sensorbox/blob/v0.6.4/include/sensorbox/stiffness.hpp). Additionally the sensor models should replace "stiffness()" with the uncertainty class to provide greater flexibility to downstream applications that may have different needs (requiring covariance rather than stiffness).
Additionally should any json function be in sensorbox, or should this be moved to convert?