[field_transformations] return field instead of `FieldPtr`

[dglaeser]

currently, the transformation return FieldPtr instances, which is used in the TransformedField class.. this should be changed. The transformations could yield actual fields, while TransformedField internally constructs a new ptr from them... this would make the transformations usable in different contexts.

[dglaeser]

We probably also don't need TransformedField. In general, this header could use some cleanup...