Align with `subsampled_view` overloads accepting `point_t` and `coord_t`
Both types are based on `std::ptrdiff_t`. Fixes compilation warnings.
Tidy up `subimage_view` and `subsampled_view` with trailing return.
Add tests for `subimage_view` in core and dynamic_image extension.
(First tests for the dynamic_image, hurray! :))
Add explicit cast to double in division operator for point<T> to
fix compilation warnings.
Split header tests accordingly i.e. test core headers as part of
core tests, numeric extension headers as part of numeric tests, etc.
It extends the convention of sub-directories already established in
`include/boost/gil` directory. It is sensible to follow it in other
areas of the source tree (i.e. `test/`, `doc/` and `benchmark/`).
Another important reason to move the tests is to enable removal of
the top-level `Jamfile` with all its definitions of test-specific
requirements.
The top-level `Jamfile` is not advised, especially if it specifies
build requirements like C++ language version.
Those affect non-tests builds e.g. documentation, causing failures
during generation of HTML documentation (leads to missing docs).
