add some old interfaces back for qubits

[Xiu-zhe (Roger) Luo]

I think we should keep nqubits and insert_qubits! instead of removing them, I suspect nqudits is very intuitive for most people when they are doing just qubits, these are relatively simple to just forward as nqubits(x) = nlevels(x)==2 ? nqudits(x) : error("...") while keep some old code keep working

[Jinguo Liu (刘金国)]

nqubits is not removed. I also added the insert_qubits! in the last commit in this PR: #334