Define diamond distance as 1/2 diamond norm of difference

[rileyjmurray]

A function call of the form diamonddist(a, b) returns the diamond norm of a - b. Our reports give things in terms of 1/2 this value. The recent QCVV tutorial paper out of the QPL actually defines the diamond distance as 1/2 the diamond norm of a-b. It would be nice if we adopted this convention.

Note: our jtracedist function already applies the 1/2 scaling, so there is precedent for adopting these definitions.