add properties 'coarse silt' and 'fine silt'

[bkempen]

There is no distinction between the coarse and fine silt fractions (as we have with the sand fractions). Please add coarse silt and fine silt to the vocab. With fine silt defined as the fraction 0.002-0.02 mm and coarse silt as 0.02-0.05 mm [there could be other definitions of coarse and fine silt as well] (relates to #113).