Add tax tables lookup to python bindings.

Add function to lookup tax tables to Book and add taxtable conversion
to base typemap.

Author: Hendrik van Antwerpen <hendrik@van-antwerpen.net>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22805 57a11ea4-9604-0410-9ed3-97b8803252fd