Massive removal of the old API.

Remove all references to the old API

• Gray level alias to uint8_t / uint16_t are removed. Use definitions in instead.
• Reference to value_traits are deprecated (use std::numeric_limits when possible)
• old IO API is removed #54 (closed)
• old Morphers are removed #54 (closed)
• unused utils are removed, other remains in <mln/core/utils/* #54 (closed)
• Old concepts removed #37 (closed)

Better project architecture #68 (closed) :

• mln/core/concepts/{image,se,...}
• mln/core/private/traits/{image,se,...}
• mln/core/private/categories/{image,se,..}