Remove support for area-dependent input attributes

These area attributes -- where an area could choose custom start, min,
max, or disabled, values for an input -- went the way of the dinosaurs
when we eliminated YAML. They were only used in the "nl-noord" area,
which has been removed.
Remove "factor" attribute from Input

Ref quintel/etsource#180
Ref quintel/etengine#565
Remove "factor" from the Input spec factory
Merge pull request #665 from quintel/factor-removal

Remove "factor" attribute from Input