Explicitly declare type of index arrays

[eschnett]

This closes issue #7 .

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• Version [e.g. Julia v1.4, Grassmann v0.5.7]

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[chakravala]

The type should not be explicitly declared, since it isn't always going to be Int. If you check with @code_warntype the method is already type stable, and sometimes it will not return an Int. For example, one might define the metric with Float64 instead of Int.

[chakravala]

Actually, I'll leave this PR open for now, but the provided solution is not correct.

I think what you need is a special case handling for when the array is empty.

The current solution will not be sufficient, since I do not wish to restrict to Int types.