Make cell size easier to change

[emmakbat]

I've been struggling a bit with how to set the cell size correctly for the 4-pt probe nanowires, the current behavior produces not great geometries with large numbers of small devices with many pads on one cell. This can be patch fixed on user side by just splitting up the devices across more cells, but I think preferably I would change the nanowire-4pt function to produce a more relaxed spacing.

However, the current utility die layout function leads to unpredictable geometry changes when simply changing max device size. E.g. the default produces this (mostly ok, but if you look closely some of the gaps in the pad routing are combined):

While if I try to solve the problem by increasing the x-dimension of max device size, it does not increase the pad spacing, and the device routing geometry becomes quite bad:

The pad spacing can't be changed from the cells level functions, and seems to be set within the utility die building function. Ideally there would be somewhat easier manual control over this, for cases where pads are unusually dense with respect to device size.

[emmakbat]

Current recommendation for manually fixing spacing issues with the current design:

• Manually force increase in number of dies the cell takes up (n+1)
• Be sure to increase both the device maximum x dimension as well as the x offset between devices by the same amount

Result:

[reed-foster]

I think this is somewhat related to #61