Let user select set-type used in tracers

[adrhill]

While internal use of BitSet for the index sets showed a large performance increase on the Brusselator example in #4, it struggles with very large input dimensionalities, as BitSet is designed for use with dense integer sets.

For very large, very sparse index sets, Set{UInt64} or implementations from other packages like SmallCollections.jl might be better suited.

Ideally, users could select the right set type for their task.

CC: @gdalle, @niklasschmitz

[gdalle]

To me this is top priority