Renamed Sigma -> strain_heating throughout.
Stronger emphasis on the fact that we require doxygen and graphviz to re-build source code browsers.
Merge branch 'dev' into hydro-beta
Clarify units of k.  Switch Nbreen example over to Hewitt et al (2012) Table 1 parameter values for alpha, beta, k, W_r = h_r.  Looks good ... though its not clear what that means.