Bug 7679: Various fixes for circulation statistics wizard

Bug 7679: Various fixes for circulation statistics wizard

- use SQL TRIM functions to avoid having '' and ' ' considered as
  different values
- use Text::Unaccent to remove accents from columns or rows values when
  accessing %table. This is required as MySQL consider as equals two
  strings that differ only by their accents when using GROUP BY clause.
- Exclude '' values from the list of columns or rows. Otherwise we could
  have a row 'UNKNOWN VALUE' and a row 'NULL' which both have the same
  values in their cells.

Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>