Notes: Use PCFieldSplitSetIS() to set a completely general set of indices as a field.
The PCFieldSplitSetFields() is for defining fields as a strided blocks. For example, if the block
size is three then one can define a field as 0, or 1 or 2 or 0,1 or 0,2 or 1,2 which mean
0xx3xx6xx9xx12 ... x1xx4xx7xx ... xx2xx5xx8xx.. 01x34x67x... 0x1x3x5x7.. x12x45x78x....
where the numbered entries indicate what is in the field.