null_to_integer_mode mode

[jacobwilliams]

Consider adding a null_to_integer_mode for integers (similar to the one for reals).

See: #517

[jacobwilliams]

The options could be:

• null_to_integer_mode
  • 0 (default) - raise an exception - what is does now
  • 1 - user specified int value for nulls
• null_to_integer_value - the value to use when null_to_integer_mode=1. could be 0 by default?