rst: added rel_name attrib to all rst segment/group nodes
- if the attrib is not empty, but if it is filled, it signals the
  RST relation that this node directly dominates
rst: added get_rst_relations()
conano: complete rewrite of conano parser to fix dominance relations
minor: add natural_sort_key to main namespace
conano: added get_conano_units() and get_connective()
conano: fixed edges for tokens in an </elements> tail
- they should go from the parent node to the token, not from element to token
rst: minor: completed docstring
util: added TokenMapper
discoursegraph: select_nodes_by_layer() now accepts multiple layers as filter
rst: fixed edge types, and segment type of parent node
workaround: can't put natural_sort_key in main namespace
main: added select_neighbors_by_layer()
rst: added get_segment_spans_from_rst_relation()
merging: added name of other graph; workaround for #96
- missing docgraph name cause paula exporter to produce
  ugly/uninformative file names
minor: added comment
fixed #39: nodes will be implicitly create when an edge is added
- the newly create node will only be part of the namespace layer
paula: made node IDs xpointer compatible (i.e. replaced ':' with '_')
get_span(): added warning for cyclic graphs
conano: added tokenize=Bool param to ConanoDocumentGraph for dot debugging
rst: fixed get_rst_relations()
- 'rst:rel_name' must not be empty