check that q is not zero

[bjarthur]

if q turns out to be zero here, then there is an uncaught exception here. a simple MWE is when length(xs) * span < 1. is it worth throwing an informative error?

julia> using Loess

julia> loess([1,2,3], [4,5,6], span=0.1)
ERROR: MethodError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
Stacktrace:
  [1] mapreduce_empty(#unused#::typeof(identity), op::Function, T::Type)
    @ Base ./reduce.jl:367
  [2] reduce_empty(op::Base.MappingRF{typeof(identity), typeof(max)}, #unused#::Type{Float64})
    @ Base ./reduce.jl:356
  [3] reduce_empty_iter
    @ ./reduce.jl:379 [inlined]
  [4] mapreduce_empty_iter(f::Function, op::Function, itr::Vector{Float64}, ItrEltype::Base.HasEltype)
    @ Base ./reduce.jl:375
  [5] _mapreduce(f::typeof(identity), op::typeof(max), #unused#::IndexLinear, A::Vector{Float64})
    @ Base ./reduce.jl:427
  [6] _mapreduce_dim
    @ ./reducedim.jl:365 [inlined]
  [7] #mapreduce#800
    @ ./reducedim.jl:357 [inlined]
  [8] mapreduce
    @ ./reducedim.jl:357 [inlined]
  [9] #_maximum#818
    @ ./reducedim.jl:999 [inlined]
 [10] _maximum
    @ ./reducedim.jl:999 [inlined]
 [11] #_maximum#817
    @ ./reducedim.jl:998 [inlined]
 [12] _maximum
    @ ./reducedim.jl:998 [inlined]
 [13] #maximum#815
    @ ./reducedim.jl:994 [inlined]
 [14] maximum
    @ ./reducedim.jl:994 [inlined]
 [15] loess(xs::Matrix{Float64}, ys::Vector{Float64}; normalize::Bool, span::Float64, degree::Int64, cell::Float64)
    @ Loess ~/.julia/packages/Loess/p2rXa/src/Loess.jl:97
 [16] loess
    @ ~/.julia/packages/Loess/p2rXa/src/Loess.jl:39 [inlined]
 [17] #loess#12
    @ ~/.julia/packages/Loess/p2rXa/src/Loess.jl:142 [inlined]
 [18] loess
    @ ~/.julia/packages/Loess/p2rXa/src/Loess.jl:142 [inlined]
 [19] #loess#13
    @ ~/.julia/packages/Loess/p2rXa/src/Loess.jl:147 [inlined]
 [20] top-level scope
    @ REPL[5]:1

julia> VERSION
v"1.9.2"

(jl_Wt4s1c) pkg> st
Status `/private/var/folders/s5/8d629n5d7nsf37f60_91wzr40000gq/T/jl_Wt4s1c/Project.toml`
  [4345ca2d] Loess v0.6.1

julia> Sys.MACHINE
"arm64-apple-darwin22.4.0"