automatically add newlines in `gautools.tools.use_gen_template`

[theavey]

Especially with the footer argument, actually adding a footer would need to be preceded and followed by a newline, which shouldn't be necessary.

Further, the documentation on this should be updated to clarify this. Again further, might be good to check for proceeding newlines in the footer argument because two newlines might be an issue for Gaussian. While updating the documentation, it might be good to make it clear that it can also be used for input modredundant or addgic, which are likely more generally common that calculating RESP charges.