[WIP] Mutable memory mapping

[evetion]

On top of #10

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 0.3%.
The diff coverage is 76.92%.

@@            Coverage Diff            @@
##           master      #12     +/-   ##
=========================================
+ Coverage   62.62%   62.93%   +0.3%     
=========================================
  Files           8        8             
  Lines         396      402      +6     
=========================================
+ Hits          248      253      +5     
- Misses        148      149      +1

Impacted Files	Coverage Δ
src/LasIO.jl	100% <ø> (ø)	⬆️
src/fileio.jl	45.45% <75%> (-0.7%)	⬇️
src/point.jl	67.27% <77.77%> (+3.27%)	⬆️

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[visr]

What shall we do with this? It looks useful to be able to replace points in a memory mapped PointVector.

As far as I see no need to also make LasPoint mutable, other than convenience. I'm worried about performance if we do that. We can always include some convenience functions for updating the classifications for instance. Also packages like https://github.com/jw3126/Setfield.jl make "modifying" immutables less of a hassle.