Support generating two-terminal footprints with IPC density argument

Fixes https://github.com/pointhi/kicad-footprint-generator/issues/421.

Branched from #438. Merge that PR first!

This adds a parameter to support IPC density settings. If the user does not enter a value for the density, nominal density is assumed and footprints are generated exactly as they are now (or, rather, would be if the aforementioned PR is merged). Giving an argument to this parameter generates footprints with correspondingly bigger or smaller pads and a suffix on the footprint name which indicates the density.

It appears the density is used during pad calculation for the handsolder footprints and I've left that alone since it gives further options for handsoldering pads. Or, those footprints can be discarded if the density argument is given and only the non-handsolder footprints added to the library. That would be my expectation.

I feel like there might be a better way to sort out the density during argument parsing than what I added at the end of SMD_package_rlc-etc.py, but my brain feels empty now so I may need to come back to this later. Or get suggestions.

I also renamed the function TwoTerminalSMDchip() to TwoTerminalSMD() because the script can generate non-chip footprints.