PLCgen: generate data types with named values for S7

Since TIA Portal V19, data types with named values (NVTs) (i.e., enums) are supported. See page 51 of this slide deck: https://cache.industry.siemens.com/dl/files/307/109821307/att_1159366/v2/TIA_Portal_V19_technical_slides_EN.pdf

I think for a user this would increase the readability of the generated code.

Addresses #679