round penwidth numbers to 3 decimals
round penwidth numbers to 3 decimals

avoids computing very small numbers written in scientific notation.

nyee/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py