[NWD] Further EPPlus fixes & improvements (take 2)

- Fix conditional formatting and add a test spreadsheet
- Remove System.Drawing dependency. With this change, the EPPlus
  library has no dependencies outside the base .NET Core library.
- Remove ability to modify most styles

Change-Id: Ida4fc1d946c8b99f7a35cab6ee1eeb503a2d8219
Reviewed-on: https://gnocchi-internal-review.git.corp.google.com/c/third_party/epplus/+/207701
Reviewed-by: Hughes Hilton <hugheshilton@google.com>