Optional rating column is added to .xlsx report, parametrization of number of...

• Added Rating and RatingRelativeToPlayer to Player class, filled in SessionTimingExtension so it's available for SessionSummaryExporter.
• Reworked the sheet columns indexing so it uses numbers instead of letters, now it's easier to calculate/parametrize the number of columns and offsets.
• Added optional Rating column in exported report.
• Styled the column similarly to how it's displayed in the program, regarding the relative ratings. The style works spotty in different programs but so is the rest of the styles sadly. Attached image is from Excel.
• Fixed occasional "System.ArgumentException: TimeSpan does not accept floating point Not-a-Number values" exception caused by player invalidating all of their laps. Now Avg and Std. Dev. will be replaced with "-" in that case.

I wasn't sure if the meaning and use case of "Rating" in the program is how I understand it, I tested it only in R3E (works) and in AC (rating columns is not added as there is no rating system). Not sure how it would work with the "other" type of ratings which are for single player races.

Thought about exporting it only when "Enable Rating" is checked but as this was primary for personal use I left the rating to always be included if present for at least one driver. Also the previous point stands - didn't want to enable something I don't use.