plsmin: Set length of minor ticks

This sets up the length of the minor ticks and the length of the terminals on error bars. The actual length is the product of the default length and a scaling factor as for character height.

def (PLFLT, input) The default length of a minor tick in millimeters, should be set to zero if the default length is to remain unchanged. scale (PLFLT, input) Scale factor to be applied to default to get actual tick length.