perf(util_array.util2d): write_txt string += performance slow for large arrays (#964)
perf(util_array.util2d): write_txt string += performance slow for large arrays (#964)

Writing large arrays (like JA in a disu file) using package_object.write_file() is
too slow (hours for a JA of size ~10M). Experimentation suggests the bottleneck is
in utils/util_array.py() in function array2string, which uses the string += "more"
and string = string + "more" methods, which scale badly with size.
Implement solution: Change string += loop to "".join(iterable) method.

Co-authored-by: Mike Taves <8d6d904860c1e25a45281c2a4beda433aa16c15f@gmail.com>

Fixes #961
jentjr/flopy,jentjr/flopy