Add place_pwlin for morphology geometry queries. (#1062)

* Add new `isometry` class representing a direct Euclidean isometry comprising a rotation and translation. Translations are always taken with respect to absolute coordinates, but rotations are composed with respect to intrinsic coordinates.
* Add new `place_pwlin` class, that takes a morphology and an isometry, and answers location queries: where in space does a particular `mlocation` lie?
* Split out (some of) the common code between `place_pwlin` and `embed_pwlin`.
* Add equality operator to `mpoint`.

This functionality is added to aid the presentation of the local field potential example.