Python feature probes and samples (#872)

Adds

* to `recipe.cpp`: 
`num_probes` and `get_probe` with helper function `cable_probe(kind, id, location)`
* `sample.cpp`: 
`sample` type with attributes `time` and `value`; a `sampler` with `samples()` and `clear()` and helper functions `attach_sampler(sim, dt)` for adding all probes as well as `attach_sampler(sim, dt, probe_id)` for adding one probe
* `morphology.cpp`: 
`location` as `arb::mlocation` with `branch` and `position` 
* corresponding docs and example
* update to new cable cell semantics in c++ validation test

fixes #762