Make 1D peek functions more flexible

[wtbarnes]

Currently the peek methods on Strand and Profile are hard-coded to plot temperature, density, pressure and velocity as a function of s. It would be nice if we could instead plot any arbitrary quantity/quantities as a function of s, similar to the way the time-distance plotting function works. This will require a significant refactor of these methods and some careful thinking about how to best prescribe the input parameters.