Add ACM2 model and usage

[hpretl]

The ACM2 model is a very compact model with just a few parameters (5 for dc). This could be a useful bridge between the black-box approach using gm/ID and the classic square-law. Another option would be the simplified EKV model, but likely ACM2 would be a bit better suited.

Reference: https://github.com/ACMmodel/MOSFET_model