Cleanup and improve

* Split header
* Detangle dependencies
* Use better names in user interface
  * `FormOperator` -> `RangeOperator`
  * `transformedForm(op, form)` -> `transformRange(form, op)`
* Remove `makeSumForm()`, using plain `SumForm(...)` does the trick due to CTAD
* Add operator `D` for computing partial derivatives as `D(u, i)`
* Further cleaup of form interface. Less magic, more explicit.
* Add support for passing more arguemnts to `RangeOperator`