All sensor functions return their major result value (or array of values)

[awickert]

This will allow functions to flexibly call one another, and will also allow us to develop a "smart" sensing system that, for example, adjusts its data logging frequency based on what it measures (following conversations last year with @gcng)

An example application of some internal processing could be with groundwater--surface water fluxes with temperature probes: could perhaps do this with a frequency-domain decomposition (http://wiki.openmusiclabs.com/wiki/ArduinoFFT) and/or by solving advection--diffusion equations with an oscillating upper boundary condition (this will be easier with faster, but higher-power-consumption, processors).