[WIP] units

[dokato]

I decided to include my first attempts to suport units in nat (#470) for reference.

I used units package and implemented assignement of units to XYZW columns of a d matrix.

Known problems:

• when using Ops it resets to unitless:

> head(pnu$d,2)
  PointNo Label             X             Y             Z         W Parent
1       1     2 186.8660 [um] 132.7093 [um] 88.20393 [um] 1.01 [um]     -1
2       2     2 187.3355 [um] 131.1558 [um] 90.59680 [um] 1.27 [um]      1
> head((pnu+3)$d, 2)
  PointNo Label        X        Y         Z         W Parent
1       1     2 189.8660 135.7093  91.20393 1.01 [um]     -1
2       2     2 190.3355 134.1558  93.59680 1.27 [um]      1

• Problems with operations that have units

> pnu+ set_units(3,'um')
Error in pnu + set_units(3, "um") : 
  non-numeric argument to binary operator
In addition: Warning message:
Incompatible methods ("Ops.neuron", "Ops.units") for "+" 

• assigning extra class doesn't really help

> class(pnu) <- c(class(pnu), "units")
> pnu
'neuron' with 180 vertices in 1 tree and additional classes 'units'
> pnu + set_units(3,'um')

• custom generic set_units.neurons doesn't seem to be recognized

> set_units(pn)
Error in UseMethod("set_units") : 
  no applicable method for 'set_units' applied to an object of class "c('neuron', 'list')"

Happy to take some pointers.

TODO:

• fix all issues above
• resample with units