Initial draft for a specialized computed_assign for xfixed

[DerThorsten]

Initial draft for a specialized computed_assign for xfixed:

• added overload for fixed/nonfixed shape via enable_if
• added tests

Any broadcasting which would require a reshaping of the tensor we assign to will lead to an broadcast error.
But there is one thing where I do not know how to solve it:

Lets say a has a shape of (2,2) and b a shape of (1,2)

  xt::xtensor_fixed<int, xt::xshape<2,2>> a = {{1,2},{3,4}};
  xt::xarray<int> b = {{5,6}};

and I do

xt::noalias(a) += b;

For such expressions, where broadcasting is involved to bring b to the correct shape, test suite only passes if I, within my implementation for computed_assign do this:

...
base_type::assign_data(e1, e2, false/*trivial_broadcast*/);
...

If I set this to true, the test suite only passes for expression where no broadcasting is involved.

I am unsure how to figure out in a proper way from within computed_assign since the shape and dim of the expressions e1 and e2 are matching.

[DerThorsten]

@JohanMabille I think I need your input here