Tag initial version of Arrowhead

[Arrowhead](https://github.com/ivanslapnicar/Arrowhead.jl) package contains routines for forward stable algorithms which compute:

* eigenvalues and eigenvectors of a real symmetric arrowhead matrices,
* eigenvalues and eigenvectors of matrices which are a rank-one modification of diagonal matrices (DPR1),
* singular values and vectors of half-arrowhead matrices.