Provide comma-delimited ASCII output

[bgctw]

Currently the output-format written by fWriteDataframeToFile is handled by fLoadTXTIntoDataframe but hard to parse by other user code, maybe in other programming languages. This is largely due to keeping legacy compatibility.
Please, provide a format that is easier to parse.

Specifically

• comma-delimited
• variable names and units in two header rows before the actual numeric data
• '%' symbol replaced with the word 'percent' to avoid some parsing problems
• no whitespaces within an ID, e.g. DateTime or Date_Time instead of Date Time.

[bgctw]

I wold rather not duplicate the variable names, but rather provide just the units in a single row above the variable names.