Add resistance matrix export capability

[atx]

I am envisioning that the user could specify "ports" via something like !padne PORT name=... at=... and then running a padne zmatrix ... command, which would export the mutual resistive coupling between the ports. This is related to #23, since both issues would benefit from the ability to "prepare" the system matrix and then only alter its right hand side, computing the individual solutions quickly.

This feature should be useful if:

• Someone would like to include these parameters in their lumped element SPICE simulation for whichever reason
• To have an automatic DRC check of the sort "resistance between those points is not more than X" (though maybe more comprehensive suite of DRC features would be better?)