gnucap-modelgen-verilog initial commits

Dear all,

We have taken the first steps towards our VerilogAMS model compiler [0].
The new code generates code for partial derivatives in the context of
analog blocks, as used in iterative solvers.

This work was funded through the NGI0 Entrust Fund, see [1]. It
concludes task 1b in [2].

Best wishes
felix

[0] https://git.savannah.gnu.org/cgit/gnucap/gnucap-modelgen-verilog.git/log/?h=deriv
[1] https://nlnet.nl/project/Gnucap-VerilogAMS/
[2] http://gnucap.org/dokuwiki/doku.php/gnucap:projects:nlnet:verilogams