Extending system

Extended Discretizer.build() interface to work with existing builders. Implemented features:

1. If builder is provided, all sites, hoppings and leads from it would be added to new builder. Moreover, new builder won't overwrite any site or hopping.
2. If symmetry or periods is provided, instead of returning lead itself, system with attached lead will be returned.
3. If, in addition, tb_connect argument is provided (of type Discretizer), instead of using hoppings from self to attach new sites to old sites, hoppings from it would be used. Useful for creation of tunnel barriers.
4. Some simple tests for it are added, possibly needs extension.