There is no need for format-specific subfolders.
There is no code in place that checks if all those folders exist and
creates them.
This change simplifies the structure to simplify running of I/O tests.
Rename images/jpg/ to images/jpeg/ in tests
The `jpeg` is name of the format used in the I/O extension and
the folders inside `images/` are named after those formats.
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).
