Bug 723051 Implement gncCustomerNextID in Python bindings.

Alter the SWIG interface file to expose the
gncCustomerNextID in Python and adds a CustomerNextID() function
in the book class similar to the InvoiceNextID and BillNextID function.

Author: Tom Lofts <dev@loftx.co.uk>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23777 57a11ea4-9604-0410-9ed3-97b8803252fd
Fix comment and trailing whitespace.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23778 57a11ea4-9604-0410-9ed3-97b8803252fd