Issue #148: Parse cost sheet from GSheet

[praqma-thi]

Warning
Work in progress

Allow for parsing auxiliary data (notably costs) from Google Sheets.
See issue #148.

[praqma-thi]

I added a class for interacting with Google Sheets through a Service Account.
It has a temporary main function showing some sample usage.

Create an instance, passing in:

• the spreadsheet ID (docs.google.com/spreadsheets/d/xxxxxxxxxxxxx_xxxxxxxxxxxxx)
• path to the service account key file (created and downloaded from Google Cloud Console)

Note: I already made a service account for us, and shared our costs sheet with it.

[kvarak]

With #177 this is no longer needed?