Add single cell example. (#703)

Adds a small self contained example demonstrating the use of SWC and morphology specifications, to fill a void that will be left when the miniapp is removed.

* Adds `single-cell` example, together with short README and example SWC file.
* Removes redundant constructors for context objects.
* Corrects and simplifies some of the context comments.
* Add a method to `mc_segment` for calculating (approximately) a lower bound on the length constant of that segment for a given frequency, for use with a NEURON 'd-lambda' style discretization rule.