feat: implement Verlet neighbour list

[V-Francois]

Summary
This PR implements a flavor of Verlet list. It addresses #63.

The VerletList object contains for each particle in the system, an array of all its neighbours (defined as all particles within the interaction cutoff + some arbitrary parameter dr).
These arrays of neighbours are updated only when a given particle moves a distance greater than dr/2, ensuring that the list of neighbours always contains all particles within the interaction cutoff radius.

Test plan
The CI was updated to check over a trajectory that the energy is identical to that obtained using different neighbour list methods.

Performances
The VerletList implementation was compared to the LinkedList one, for a system of ortho-terphenyl molecules, at density 1.2 and temperature 2.
In the attached plot, the simulation time is reported for different values of the parameter dr. The dashed line at the top is the timing with the LinkedList implementation. For reference, the interaction cutoff is 1.23.
The timing is done once for each Verlet list, and 4 times for the LinkedList, with values ranging from 75s to 105s.
There's quite a lot of variability between repeats, but we still have a ~2x speedup for this system, for 0.3 < dr < 0.6.

[codecov]

Codecov Report

❌ Patch coverage is 96.59091% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/neighbours.jl	97.50%	2 Missing ⚠️
src/ParticlesMC.jl	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/IO/IO.jl	69.83% <100.00%> (+0.12%)	⬆️
src/atoms.jl	96.87% <100.00%> (ø)
src/molecules.jl	71.11% <100.00%> (ø)
src/ParticlesMC.jl	8.91% <0.00%> (+1.98%)	⬆️
src/neighbours.jl	82.24% <97.50%> (+9.44%)	⬆️

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