Support indexing of Maxima arrays

[nsmith5]

As noted in issue #25, support for indexing of Maxima arrays would be handy. Would look something like,

getindex(g::MExpr, i) = MExpr("$g[$i]") |> mcall

g = m"[x, y, z, w + y]"
g[4]   # == m"w + y"