[Review] LICENSE file does not contain actual license text

[espottesmith]

Very small issue: License.txt makes reference to the GPLv3 license but does not actually reproduce the text. For acceptance into JOSS, the contents of the license should actually be provided (see here).

[chmarti1]

Quite right. This was the way I originally assembled the package back in 2015, and I haven't really revisited it since. Easily fixed.