Enh component active pixel

Main changes:

* When pixel_level_switching is active, it is not longer necessary to call fetch/store in order to fetch and store the value from/in the array.
* Some modifcations to the User Guide.
* New tests that helped find and fix a couple of bugs.
* Rename `enable_pixel_level_switching` -> `active_is_multidimensional`
* Rename `active_map` to `_active_array`.