Slicing with caching and multiple indices

[trulsf]

There is a problem with slicing when caching is activated, having three or more indices and slicing on two of them.
The following is a MWE

I = Set(["A", "B", "C", "D"]) 
K = Set(1:1000)

m = Model()
@variable(m, y[I, I, K] >= 0, container = IndexedVarArray)
for _ in 1:1000
    i, j, k = rand(I), rand(I), rand(1:1000)
    unsafe_insertvar!(y, i, j, k)
end

SparseVariables.select(y, :, :, 1000)

Changing the return value of _dropslices_gen to return :(tuple($(exs[end])...)) seems to fix the problem, but may not be the best fix.

[trulsf]

There is also an issue with slicing on all indices, that currently fails due to the pattern being mapped to a cache index of 0. It should preferably just return all elements without any caching, or maybe have an offset of one.