Add support for cmyk and greyscale

[EmilDohne]

• Add support for cmyk and greyscale with tests
• [CI] Build wheels in parallel
• Add python tests
• Automatically forward mask in constructor of Layer from PhotoshopFile
• Add setMask() function on all layers
• Add findBitdepth function on PhotoshopFile which will give you the bitdepth of a file programmatically
• Add flatLayers() function in C++ that will give you a view over the flattened layer hierarchy

Python:

• Add rescale_canvas example
• ImageLayer: add num_channels and channels property
• Layer: rename -> layer_mask property to be mask and make it read-writeable
• Layer: add is_visible property
• Layer: add is_locked property
• Layer: add has_mask() function
• LayeredFile: add layers_flat property for iterating the flattened layer hierarchy

General:

• Restructure python declaration to be more error safe and with better logging