Sparse version of the sampler

[ameligrana]

Our sampler doesn't support sparse indices

julia> using DynamicDiscreteSamplers

julia> ds = DynamicDiscreteSampler();

julia> push!(ds, 10^10, 1.0)
ERROR: OutOfMemoryError()
Stacktrace:
 [1] GenericMemory
   @ ./boot.jl:516 [inlined]
 [2] array_new_memory
   @ ./array.jl:1058 [inlined]
 [3] (::Base.var"#133#134"{Vector{Tuple{…}}, Int64, Int64, Int64, Int64, Int64, Memory{Tuple{…}}, MemoryRef{Tuple{…}}})()
   @ Base ./array.jl:1129
 [4] _growend!
   @ ./array.jl:1116 [inlined]
 [5] resize!
   @ ./array.jl:1450 [inlined]
 [6] push!(ns::DynamicDiscreteSamplers.NestedSampler5{64}, i::Int64, x::Float64)
   @ DynamicDiscreteSamplers ~/.julia/dev/DynamicDiscreteSamplers/src/DynamicDiscreteSamplers.jl:407
 [7] top-level scope
   @ REPL[3]:1
Some type information was truncated. Use `show(err)` to see complete types.

I propose to add it with a keyword, like DynamicDiscreteSampler(; sparse=false/true) because supporting it by default would cause overhead