new function to set indicators in MARC fields

new function to set indicators in MARC fields

migration_tools.set_leader(marc, tag, indicator_pos, value):

Set indicator value of a specified MARC field.
The first argument is a MARCXML string.
The second argument is a MARC tag.
The third argument is the indicator position, either 1 or 2.
The fourth argument is the character to set the indicator value to.
All occurences of the specified field will be changed.
The function returns the revised MARCXML string.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>