Cannot create configurations with ridiculously high main quantum numbers

[jagot]

julia> c"1s2 60s2"
ERROR: LoadError: MethodError: no method matching getindex(::Nothing, ::Int64)
Stacktrace:
 [1] parse_orbital(::Type{Orbital}, ::SubString{String}) at /Users/jagot/.julia/dev/AtomicLevels/src/configurations.jl:173
 [2] _broadcast_getindex_evalf at ./broadcast.jl:582 [inlined]
 [3] _broadcast_getindex at ./broadcast.jl:555 [inlined]
 [4] getindex at ./broadcast.jl:515 [inlined]
 [5] copyto_nonleaf!(::Array{Tuple{Orbital{Int64},Int64,Symbol},1}, ::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1},Tuple{Base.OneTo{Int64}},typeof(AtomicLevels.parse_orbital),Tuple{Base.RefValue{UnionAll},Base.Broadcast.Extruded{Array{SubString{String},1},Tuple{Bool},Tuple{Int64}}}}, ::Base.OneTo{Int64}, ::Int64, ::Int64) at ./broadcast.jl:931
 [6] copy at ./broadcast.jl:794 [inlined]
 [7] materialize at ./broadcast.jl:756 [inlined]
 [8] parse(::Type{Configuration{Orbital}}, ::String) at /Users/jagot/.julia/dev/AtomicLevels/src/configurations.jl:191
 [9] @c_str(::LineNumberNode, ::Module, ::Any) at /Users/jagot/.julia/dev/AtomicLevels/src/configurations.jl:196
in expression starting at REPL[31]:1

It is however possible to create a single orbital with n=60:

julia> o"60s"
60s

[mortenpi]

Hmm, it didn't automatically close. Fixed by #20.