Remove "factor" attribute from Input

It turned out that the Input "factor" attribute *was* used somewhere: to change the start value of an input. But only if the start value was set using a GQL query. Because that makes sense. :confused:

* Start values set using GQL are no longer affected by "factor",
* "factor" attribute removed,
* Removed support for area-dependent attributes (old area YAML files could specify custom values for inputs)

This must be merged at the same time as the corresponding ETSource pull request: quintel/etsource#704.