Towards resolution of issue #283.

* Replace probe/sample infrastructure with new API as outlined in the `sampling_api.rst` documentation.
* Separate `cell` probe information from `cell` description object.
* Add `--list` option to `tsplot`, to summarize available time series for plotting together with their metadata.
* Add `--sample-dt` option to validation tests.
* Change validation time series comparison behaviour: linf distance metric now estimated by comparing reference data points against linearly interpreted simulation samples, rather than the other way around.
* Add utility class `any_ptr` which allows lightweight and type-checked access to a type-erased pointer.
* Replace `singleton_recipe` with two simple recipe base classes used for unit and validation tests, in `tests/simple_recipes.hpp`.
* Simplify RSS cell implementation.
* Add statistical functions for testing of probabilistic methods in `tests/unit/stats.hpp` (and .cpp):
   * simple summary stats class;
   * Kolmogorov–Smirnov one-sided statistic and CDF;
   * approximate Poisson CDF.
* Simplify and refactor miniapp `sample_trace` code.
* Add new utility class `handle_set` for managing a collection of integer handles. (Near-simplest implementation.)
* Relax const-ness constraints on `transform_iterator`.