AtomsBaseTesting.test_approx_eq behaviour for identical systems with different units

[Alexsp32]

AtomsBaseTesting.test_approx_eq does not consider two identical systems using different units (e.g. Bohr and Å) as equal.
This behaviour is different to previous versions and seems to be related to the addition of cell tests in #121.

Example:

using AtomsBase
using AtomsBaseTesting
using Unitful, UnitfulAtomic
using Test

box = 10.26 / 2 * [[0, 0, 1], [1, 0, 1], [1, 1, 0]]u"bohr"
box_A = [[uconvert.(u"Å", i[j]) for j in 1:3] for i in box]
silicon = AtomsBase.periodic_system([:Si =>  ones(3)/8,
                           :Si => -ones(3)/8],
                           box, fractional=true)
silicon_A = AtomsBase.periodic_system([:Si =>  ones(3)/8,
                           :Si => -ones(3)/8],
                           box_A, fractional=true)

AtomsBaseTesting.test_approx_eq(silicon, silicon_A)

Fails with:

  Expression: typeof(cell(s)) == typeof(cell(t))
   Evaluated: AtomsBase.PeriodicCell{3, Unitful.Quantity{Float64, 𝐋, Unitful.FreeUnits{(Å,), 𝐋, nothing}}} == AtomsBase.PeriodicCell{3, Unitful.Quantity{Float64, 𝐋, Unitful.FreeUnits{(a₀,), 𝐋, nothing}}}

[mfherbst]

Indeed, that makes sense. Thanks for flagging.