einsum for xtensor

This project implements the numpy einsum function for the xtensor project. (It borrows heavily from the numpy implementation.) Currently, the syntax looks like this

enum labels: int {
  _ = 0,
  I,
  J,
  K
}

auto x = einsum<subscripts<I,J>,subscripts<J,K,_>>(_).eval<subscripts<K,_,I>>(y,z)
auto w = einsum<subscripts<I,J>,subscripts<J,K,_>>(_).eval<implicit_out>(y,z)

for the equivalent numpy statements

x = np.einsum('ij,jk...->k...i', y, z)
w = np.einsum('ij,jk...', y, z)

Please include the main/include directory to test this in your project.

To run the tests, set the $EINSUM_TEST_HOME environment variable to the full path to the test directory on your machine and then run make && ./test/main.