Performance improvements

[SebastianM-C]

Hi! I noticed that there some performance improvements that can be made. For example due to atoms::Vector{<:Any} a lot of dynamic dispatch was occurring inside force calculations. Adding another type parameter solved the problem and provided a ~40x speedup for Lenard-Jones interactions. I think further improvements can be made.

[jgreener64]

This was a regression introduced in https://github.com/jgreener64/Molly.jl/commit/97ffa4c3dd3581b832d8cbcd17b2b94306cab5fd, thanks a lot for the fix.

Shows I need to do performance testing as part of CI. I'll look into a simple benchmark suite.

[coveralls]

Pull Request Test Coverage Report for Build 62

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 79.348%

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Totals
Change from base Build 61:	-0.5%
Covered Lines:	219
Relevant Lines:	276

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💛 - Coveralls