Add functions to read and write 'fhx' objects to CSV or JSON files

[brews]

Add functions to read and write 'fhx' objects to CSV or JSON files.

This idea came out of a discussion in #115.

[brews]

CSV format makes the most sense because R is best with "rectangular" data rather than blob-like data objects.

[brews]

Now that I think about it. We should already have this in v0.5.0 because of #120 .

Example to write to CSV:

library(burnr)
data(lgr2)

write.csv(lgr2, "lgr2.csv")

and we can read again with:

x <- as.fhx(read.csv("lgr2.csv"))

Tah-dah!

as.fhx() currently discards any cruft rows and only creates an fhx object from the columns it needs so we don't need to worry about saving and reading row.names, here.

Marking as invalid and closing this issue.