16 use ephemeridesjl as backend in tests

[MicheleCeresoli]

Closes #16.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3842cb8) 86.90% compared to head (418bb0a) 86.80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #20 +/- ## ========================================== - Coverage 86.90% 86.80% -0.10% ========================================== Files 30 30 Lines 2023 2023 ========================================== - Hits 1758 1756 -2 - Misses 265 267 +2 ``` | [Files](https://app.codecov.io/gh/JuliaSpaceMissionDesign/FrameTransformations.jl/pull/20?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSpaceMissionDesign) | Coverage Δ | | |---|---|---| | [src/Frames/axes.jl](https://app.codecov.io/gh/JuliaSpaceMissionDesign/FrameTransformations.jl/pull/20?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSpaceMissionDesign#diff-c3JjL0ZyYW1lcy9heGVzLmps) | `90.28% <ø> (-0.58%)` | :arrow_down: | | [src/Frames/points.jl](https://app.codecov.io/gh/JuliaSpaceMissionDesign/FrameTransformations.jl/pull/20?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSpaceMissionDesign#diff-c3JjL0ZyYW1lcy9wb2ludHMuamw=) | `92.08% <ø> (ø)` | | | [src/Frames/types.jl](https://app.codecov.io/gh/JuliaSpaceMissionDesign/FrameTransformations.jl/pull/20?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSpaceMissionDesign#diff-c3JjL0ZyYW1lcy90eXBlcy5qbA==) | `67.56% <ø> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaSpaceMissionDesign/FrameTransformations.jl/pull/20/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSpaceMissionDesign)

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