API & Slider: Coord->Component

[ax3l]

Follow-up to #148: the "coord" option in the field API and interactive ipywidget should be renamed to component (or comp) since a component of a field does not necessarily reflect a certain quantity along a vector component of the chosen geometry. The current naming can be confusing, e.g. for tensor fields.

It is also the name used in the standard for the same reason, record and record component.

[ax3l]

For the time being, one can also add that option to the API with same functionality and print a deprecated warning if people still use the coord option. This would even be API compatible and one could remove it later on.

Renaming the slider label is possible at anytime.