CSV export - add some user configuration options

User euqip provided some helpful suggestions regarding my original
changes.  This commit incorporates some of these.

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
3c5751a
2. The thousands separator is no longer used.  It is effectively
specified as 'nothing'.  Previously it defaulted to ','.