increase guess_max in read_excel to avoid values been forced to boolean

[orichters]

• I had a long snapshot file from IIASA, where the yearly MAGICC temperature data was below all the 5-year IAM results
• reading this file lead to 2031, 2032 etc. temperature data being forced to 1 or 0
• I spent quite some time making a minimum working example, because the error kept going away when I reduced the file length.
• Reason: guess_max of read_excel which tries to guess column type defaults to 1000, and interprets empty cells as "boolean" 😝
• you can set it to Inf, but then it complains: guess_max is a very large value, setting to 21474836 to avoid exhausting memory
• So let us use 21474836 instead
• I could not see any difference in loading time