Add error message to get_index_n/p if called without a value list being set.

This may happen if a LeqDomain domain is created, e.g. (x <= 5) == z, and equal() is called without the order encoding of 'x' being enabled.
For now, in the unit tests, it will enable the order encoding where needed, but we should find a better solution for parameterized test-cases in future.
Small bit of tidy in SatWrapper deleting old commented code.
Add support for MIP encoding of modulus, absolute and negation.

Squashed commit of the following:

commit f14c51c9107e7397bba503a686c5721fdc087324
Author: 9thbit <9thbit@gmail.com>
Date:   Thu Jun 27 15:00:17 2013 +0100

    Add support for MIP encoding of modulus, absolute and negation.

commit 3db04bf1235caa60d4ceb3c0cb6b1091976fc64f
Author: 9thbit <9thbit@gmail.com>
Date:   Thu Jun 27 14:57:08 2013 +0100

    Small fixes for some debug statements and error output.