CSV export - add some user configuration options

Add configuration of parameters for controlling some aspects of CSV data
export to default_settings.php.

1. number of decimal places
2. decimal place separator
3. field separator

Notes:
1. This updates/enhances the original changes I proposed to
number_format() in commit/pull request
3c5751a7e21fab8798d34a2591c949b87489bbcd
2. The thousands separator is no longer used.  It is effectively
specified as 'nothing'.  Previously it defaulted to ','.