[NWD] Further EPPlus fixes & improvements.

- 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: I54fb15b85454fb6e4339d4d65979416b52feee68
Reviewed-on: https://gnocchi-internal-review.git.corp.google.com/c/third_party/epplus/+/207761
Reviewed-by: Hughes Hilton <hugheshilton@google.com>