Add AbstractIndex

[blegat]

Needed for https://github.com/JuliaPolyhedra/ParmaPolyhedra.jl/ The tests are broken because of https://github.com/JuliaLinearAlgebra/GenericLinearAlgebra.jl/issues/121

[andreasnoack]

Upstream issue is fixed

[blegat]

@andreasnoack Thanks!

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (b40c1be) 88.98% compared to head (89b6a00) 88.98%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #322 +/- ## ======================================= Coverage 88.98% 88.98% ======================================= Files 37 37 Lines 3167 3167 ======================================= Hits 2818 2818 Misses 349 349 ``` | [Files](https://app.codecov.io/gh/JuliaPolyhedra/Polyhedra.jl/pull/322?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPolyhedra) | Coverage Δ | | |---|---|---| | [src/indices.jl](https://app.codecov.io/gh/JuliaPolyhedra/Polyhedra.jl/pull/322?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPolyhedra#diff-c3JjL2luZGljZXMuamw=) | `100.00% <100.00%> (ø)` | | | [src/iterators.jl](https://app.codecov.io/gh/JuliaPolyhedra/Polyhedra.jl/pull/322?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPolyhedra#diff-c3JjL2l0ZXJhdG9ycy5qbA==) | `99.24% <100.00%> (ø)` | | | [src/lphrep.jl](https://app.codecov.io/gh/JuliaPolyhedra/Polyhedra.jl/pull/322?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPolyhedra#diff-c3JjL2xwaHJlcC5qbA==) | `90.47% <ø> (ø)` | |

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