[NWD] Move EPPlus classes to a new namespace.

Also rename the assembly.

This will allow us to have both the legacy and new EPPlus version
referenced in AppSheet for a slow rollout.

Change-Id: Idf4c4d2423041d626d6e4185d7de53b7b786164f
Reviewed-on: https://gnocchi-internal-review.git.corp.google.com/c/third_party/epplus/+/229480
Reviewed-by: Hughes Hilton <hugheshilton@google.com>