fix output parsing

This branch (with a very bad name) introduces principally the following changes:

1. Image.hpp

• modification of save method to

    virtual IMAGE_LOAD_SAVE_FLAGS save(std::string const &file_path, std::function<void(int)> progress = [](int) {},
                                       std::string additional_comments = "" ) const = 0;

• Storage of metadata as associative map

protected:
    std::map<std::string, std::string> _metadata;

2. All classes that inherit Image have been updated consequently

3. EXR, ENVY and ART which supports metadata are now using the new interface and data structure

4. Unit tests have been updated to test for loading and saving of metadata

5. By default, Malia now saves images in exr format (for malia_rgb) and in ENVY format (for malia) when no output file is specified.

6. Malia will append _number_of_samples_per_pixelspp to the output image filename
   when the requested filename has PNG extension (e.g., my_image_100spp.png)

Note: More code refactoring in ENVY and ART classes could probably be made to manage metadata...