[MAINT] Divide 'Multiarray.hpp' into several headers.

[oddkiva]

As the title says, we divide the header 'MultiArray.hpp' into several header files:

• 'ElementTraits.hpp'
• 'MultiArray.hpp'

In the wake of this PR, we also did the following:

• Raise a runtime exception when using the assignment operator to wrapped data instead of exiting the program.
• Add unit test for that.
• Add missing copyright notices to some recently added files.

[oddkiva]

It seems there is room for improvement regarding the assignment operator of the 'MultiArray' class (cf. issue #108).

[oddkiva]

Travis is happy. Merging.