Provide method get_value() for views_handler_field_math

We should provide method get_value() for class views_handler_field_math because other module will use this raw (float) value to do other things.
Because class views_handler_field_math extend class views_handler_field_numeric, we should remove method render() and inherit it from class views_handler_field_numeric.

Class views_handler_field_numeric will call method get_value() and render this raw value to formatted number.
http://cgit.drupalcode.org/views/tree/handlers/views_handler_field_numeric.inc#n109