zerodim, nid, re-organization

this pull request brings some big nice changes to bertini2:

std::make_shared<> has been largely replaced with calls to MakeVariable, etc, allowing for future implementation of a management scheme for things like nodes, etc, using the pools.
zerodim algorithm is looking pretty good. has methods for accessing the settings, etc.
basic skeleton for NID is implemented.
witness set implemented, with policies for storing the points, etc.

bertiniteam / b2