Define `alias` function to make it simpler to define gate aliases

[mtfishman]

See here for example: https://github.com/ITensor/ITensors.jl/blob/fb9b73a08513bd677fa9e0d81c81ea17e4582c24/src/physics/site_types/electron.jl#L79-L88

This should allow us to simplify the code. For example, we could define a single fallback definition:

gate(gn::GateName; kwargs...) = 
  gate(alias(gn); kwargs...)

which would automatically look for an alias of a gate name, so that we don't have to explicitly define overloads of gate for every new alias.

[GTorlai]

Good idea, though I'm not sure how we would avoid this clashing with the basis rotation function"

function gate(::GateName{gn}; kwargs...) where {gn}
  gn_st = String(gn)
  if startswith(gn_st, "basis")
    GN = GateName(replace(gn_st, "basis" => ""))
    return eigenbasis(GN; kwargs...)
  end
  return error(
    "A gate with the name \"$gn\" has not been implemented yet. You can define it by overloading `gate(::GateName\"$gn\") = [...]`.",
  )
end

Or maybe I am missing some fallback mechanism?

[mtfishman]

We could put the call to gate(alias(gn); kwargs...) within that function, but that could lead to infinite loops so we would need to design it carefully.