Add point2<T> alias template for backward compatibility with Boost <=1.68.
Replace multiple point_t aliases of point<ptrdiff_t> with single defined
in point.hpp. The point_t is common used to represent dimensions.
Replace many uses of point<ptrdiff_t> with point_t.
Apply reformatting around point2 changes to respect the line length limit.
Follows up discussion in #154
Group include directives, sort within group:
* In headers of GIL core and extensions:
1. boost/gil/extension/*
2. boost/gil/*
3. boost/*
4. C++ standard library headers
* In programs:
1. boost/gil/*
2. boost/*
3. C++ standard library headers
4. "xxx.hpp" for local headers
Add basic guidelines to CONTRIBUTING.md.
Add/Remove #include <boost/config.hpp> or std headers un/necessary.
Rename gil_concept.hpp to concepts.hpp.
Remove gil_all.hpp - we already have all-in-one boost/gil.hpp.
Tidy up and unify copyright and license header.
Tidy up formatting and excessive whitespaces in some comments.
Remove Doxygen block with file description, author, date, etc.
Remove dead or commented pragmas and directives.
Trim trailing whitespaces.
