Add test suite for color_convert and default_color_converter

[mloskot]

We need to start a detailed test cases coverage for color conversion algorithms:

• Add test/core/pixel/color_convert.cpp
• Use Boost.Test to define test cases using BOOST_AUTO_TEST_CASE or BOOST_AUTO_TEST_CASE_TEMPLATE
• Add test cases for each pre-defined specialisations of converters
• Covert byte-based as well as bit-aligned and packed pixel types

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It's a good idea to exercise the color_convert and default_color_converter using available interfaces, for example

gil::rgb8_pixel_t red(255, 0, 0);
... red2;

// three equivalent conversions
gil::color_convert(red1, red2);
gil::default_color_converter()(red1, red2);
red2 = gil::color_convert_deref_fn<gil::rgb8_ref_t, ...>()(red1);

[pankaj-2407]

Hi, I would like to work on this issue.

[lpranam]

Feel free to take on this. No need for permission. :D

[pankaj-2407]

Thankyou @lpranam