Read in a weight-at-age data file as a data frame
Value
Returns a data frame with a variable number of columns based on the
number of ages that are included in the file. Though, the first columns
will always be year, seas, sex, bio_pattern, birthSeas, and fleet.
The seventh column will be age zero.
The last or next to last column will be the maximum age included
in the weight-at-age data. For Stock Synthesis versions 3.30 and greater,
the last column will be a column of comments.
NULL is returned if file does not exist or if the file does exist but
it is empty. This behavior is different than other SS_read* functions that
error if either of the previous checks are TRUE. Thus, a complicated check
involving tryCatch() is used around readLines() to allow for returning
NULL rather than stop(). Additionally, this check accommodates a URL
for file.