Minimum fractions

[jtgilbert]

• Need an input parameter that specifies a minimum fraction (e.g., 0.01) for the finer fractions (e.g. 1 - 6 mm)
• Need to do a check to make sure that the fraction bin that D84 falls into != 0 and that there is at least some minimum fraction (e.g. 0.01) of the next bin above D84