FinanceDatabase 2.0.0

I've improved this database not only by increasing the amount of symbols (from 180k to 300k) but also:

• Approximated the The Global Industry Classification Standard (GICS®), a standard used for sectors and industries everywhere. Note that this was approximated and therefore no actual data is collected. Furthermore, not all categories are included.
• Updated and removed tickers that either no longer exist or had outdated information.
• Made the package itself object orientated making data collecting and searching much more efficient and logical. (shoutout to Colin Delahunty for the help here too)
• The database initially featured thousands of JSON files. At the time it made sense also given my rather novice background in programming. However, a much more efficient (and manageable way) is to work with CSV files. So instead, one CSV file per asset class
• Due to using CSV files, it becomes really easy to update accordingly.
• To make loading data itself still quick, it automatically compresses the data so that loading in data is not slowed down by using a format that is more easy to update.
• Updated the README, Contributing Guidelines and overal documentation.