ENH: support preloading fields for setExpr* utilities (#2014)

- can use either command-line option "-load-fields" or dictionary
  entry "readFields" to specify field names to be preloaded.

  Essentially the same functionality as with a readFields function
  object but with a lot less typing.

- tutorial examples provided by Ryan Danks <ryan.danks@rwdi.com>
  illustrate using setExpr* utilities to calculate a quantity
  as a post-processing step.