Replace qpp with xtensor (SK)

[chinyi0523]

1. Supporting the function in qpp

• [x] trace
• [x] eigen
• [x] determinant

  List

  qpp	xtensor	qsyn support
  real	xt::real(a)	real
  imag	xt::imag(a)	imag
  conjugate	xt::conj(a)	conjugate+transpose = adjoint
  transpose	xt::transpose(a, {1, 0, 2})	transpose
  adjoint		adjoint
  determinant	xt::linalg::det(a)	TODO
  trace	xt::linalg::trace(a)	TODO
  eigenvectors, eigenvalues	xt::linalg::eig(a)	TODO
  read matrix		tensor_read
  write matrix		tensor_write
  disp		print

2. Replace qpp with xtensor and move the file into tensor/