Warn if eltype positions/velocities are not isbits

[ejmeitz]

From #162

[codecov[bot]]

Codecov Report

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

Comparison is base (bc319c2) 72.18% compared to head (953432e) 72.13%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #166 +/- ## ========================================== - Coverage 72.18% 72.13% -0.05% ========================================== Files 35 35 Lines 5274 5276 +2 ========================================== - Hits 3807 3806 -1 - Misses 1467 1470 +3 ```

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[jgreener64]

Great, maybe just change Vector{SVector{}} to a vector of SVectors since we also allow CuArrays.

[ejmeitz]

Done