[ENHANCEMENT] Maybe define `contractadd[!]` as an alternative to `contract[!]` with `β != 0`

[mtfishman]

Maybe define contractadd[!] as an alternative to, or replacement for, contract[!] with β != 0, similar to the muladd! interface of ArrayLayouts.jl. That makes it clearer it is performing both a contraction and an addition.

[mtfishman]

Closed by #82.