Correctly handle CSV files with a single separator throughout

[keith-hall]

fixes #3127 and fixes #2078

better auto-detection of CSV delimiter

• files with a tsv extension are automatically detected as tab delimited
• other files parsed as CSV go through the following steps:
  • if the first line contains at least 3 of the same separator, it uses that separator as a delimiter
  • if the first line contains only one supported separator character, it uses that separator as a delimiter
  • otherwise it falls back to treating all supported delimiters as the delimiter

supported delimiters, in precedence order:

• comma ,
• semi-colon ;
• tab \t
• pipe |