Arbitrary scanner arguments; log attribute; NOT backwards-compatible

(Fixed a bug in v2.0, amended the commit and deleted the release)
NOT backwards-compatible: the old step methods (and scanner functions) took only one extra optional NodeType argument, while the current ones take multiple ones, of which the first is now a List of NodeType.

Examples of call changes:

# OLD parallel or consecutive steps methods syntax:
gsm.consecutive_steps(['Distribution', 'Family Implementation'])

# NEW versions, named and unnamed (translating the above involves only the first of 4 optional arguments):
gsm.consecutive_steps(dict(node_types = ['Distribution']), dict(node_types = ['Family Implementation']))
gsm.consecutive_steps([['Distribution']], [['Family Implementation']])


# OLD single step syntax
gsm._scan('Method Function')

# NEW
gsm._scan(['Method Function'])