Prevent mixed CSV columns

**Sebb** ([Bug 59000](https://bz.apache.org/bugzilla//show_bug.cgi?id=59000&redirect=false)):
JMeter by default appends data to an existing response data file.

If the columm list changes this causes problems for CSV files.

When appending to an existing CSV file, the code should check if the settings are compatible.

If not, then some action should be taken, e.g. abandon the test or rename the output file so the existing file is not compromised.

If the existing file has a header line, then a simple comparison of the text is sufficient to determine if the column settings are equal.

For files without headers, the code can check if the column count is the same.
It can potentially also check whether the expected columns are numeric.

OS: All