* preserve the sum of certain tags (`step_count`, `parking:*:capacity`) during _join_ operation
* preserve total value of `parking:*:capacity` tags during _split_ operation by distributing it proportionally to the resulting ways
* the abstract osm entity now accepts a list of tags to override during the merging, but otherwise is agnostic about how tags can be merged: the concrete merging resolution might depend on the concrete action that was performed
* add validation & fix for way vertices limit imposed by OSM API (try to choose splitting locations at existing intersection vertices if possible)
* fix splitting of closed ways at two or more nodes:
this bug resulted sometimes in one extra split point in the result, or one of the split vertices to be left unsplit
instead of fully re-sorting the whole relation every time a member is split, perform a local operation: This works under the assumption that the relation is already sorted properly. The new way is inserted into the relation before or after the existing member, depending on how the old/new way connect to their neighboring members.
for cases where two ways form a loop, a additional look-ahead is implemented to disambiguate the order
Depending on which way is the longest, the new way is inserted into the
relation before the existing one. This case must be explicitly handled
for the relation to remain correct.
if a way is closed, iD needs to choose a second node to
split the way at.
This algorithm looks for a node that is both far away from
the initial node in terms of way segment length and nearby
in terms of beeline-distance. This assures that areas get
split on the most "natural" points (independent of the number
of nodes).
For example: bone-shaped areas get split across their waist-
line, circles across the diameter.
