new Strassen methods: split_line, multiple_split_line

With a new test script strassen-coreheavy-split.t.
avoid "splice() offset past end of array" warning
new test case: multiple split on one record
implementation of callbacks on split_line
insert_point shortcut
Merge branch 'strassen-coreheavy-split'
Strassen::GPX: preserving WGS84 coordinates in gpx2bbd is now possible

See the newly added testcase in t/strassen-gpx.t for a howto.
gpx2bbd: new option -wgs84
new script bbd_splitlines_by_points
Strassen::GPX: -as => 'multi-tracks'

With this option one can create multiple <trk> elements, instead of
just one <trk> element and multiple <trkseg>.
bbd2gpx: note in usage that -as multi-tracks is now possible
usage example for ddd -> dmm conversion
bbd_splitlines_by_points documentation: use new "-as multi-tracks" option
cease minor warning in test