[point iterator] minor stylistic fix
[point iterator][concatenate iterator] make the unit test more elaborate: add tests for vector-vector and list-list container conbinations
[point iterator] add tests for const/non-const iterator equality/inequality
[point iterator][flatten iterator] replace enable_if mechanism in copy constructor by static asserts;
add static assert to assignment operator; make dereference inline;
[point iterator] replace enable_if mechanism by static assert;
[point iterator][concatenate iterator] replace enable_if mechanism by static assert; add templated assignment operator; make dereference and increment inline;
templatize equal for comparison against other concatenate iterator (needed for interoperability between const/non-const versions);