Bug 11413: Fix field_numbers

Bug 11413: Fix field_numbers

This fix is a global fix for the MarcModificationTemplate feature.
Some unit tests were missing and some behaviors were wrong.
For instance, if you tried to update a non existent field, the script
crashed.

The following line was completely stupid:
    if $from_field ne $to_subfield

The field_number equals 1 if the user wants to update the first field
and 0 for all fields.

The field_numbers (note the s) variable contains the field numbers to
update. This array is filled if a condition exists (field exists or
field equals).

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>