Example dataset for read_table()

[jennybc]

massey-rating.txt and epa78.txt aren't great datasets to use in the examples of read_table() in its current form (there are problems with variable number of fields). The use made sense in the past when read_table() worked more like what is now read_fwf(), e.g. it would calculate a fixed width and use it for all the fields.

A short term measure is to demo read_table() with fwf-sample.txt, but longer term it would be good to add a new example file that is not formatted with fixed-width fields, but that is whitespace delimited.