Change the default DDA formulation

[myurkin]

With development of analytical IGT (#299), it can be used in all cases, while in many cases it outperforms the default LDR (with point dipoles). Hence, we may set is as the default formulation.

However, it requires a lot of testing and changes to example simulations, tutorials, etc. In some sense it will break backward compatibility and wreak havoc in tests/2exec. So this should probably be done before the release, not to interfere with testing of other features.

[myurkin]

This is especially important for rectangular dipoles, since using the default formulation (point dipoles) there is very far from optimal (for most practical cases).