WIP: Unitful arrays

[cstjean]

I believe that the datastructure is fine, but I am unhappy with everything else. My use case is with StaticArrays, and there doesn't seem to be a nice way of accommodating both Array and SArray objects in the unit conversion routines. At least, not without also requiring StaticArrays, and at that point, we should probably start a new package entirely. Suggestions are welcome.

Update: it seems that there's a solution to this problem after all. To be implemented.

cc. @mschauer

[coveralls]

Coverage decreased (-0.1%) to 74.398% when pulling 3cdd893 on cstjean:unitful-arrays into c76f881 on ajkeller34:master.

[codecov]

Codecov Report

Merging #191 into master will decrease coverage by 0.07%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
- Coverage   74.65%   74.58%   -0.08%     
==========================================
  Files          14       15       +1     
  Lines         947      968      +21     
==========================================
+ Hits          707      722      +15     
- Misses        240      246       +6

Impacted Files	Coverage Δ
src/Unitful.jl	100% <ø> (ø)	⬆️
src/arrays.jl	77.27% <77.27%> (ø)
src/logarithm.jl	66.5% <0%> (-0.66%)	⬇️

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[alhirzel]

I recently started to encounter issues with arrays of Unitful values, and wanted to present one example. @cstjean, do you think this would be addressable?

# fit a quadratic to three points, y = 5x^2 + 0.1, x in meters, y in seconds
using Unitful.DefaultSymbols

x = [0.0s  1.0s   2.0s]'
y = [0.2m  5.2m  20.2m]'

A = [x[1]^2 x[1]^1 x[1]^0
     x[2]^2 x[2]^1 x[2]^0
     x[3]^2 x[3]^1 x[3]^0]

# ideally, units would be calculated rather than assigned here
coeffs = ustrip.(A)\ustrip.(y) .* [m/s^2; m/s; m]
@assert all(ustrip.(abs.(coeffs - [5.0m/s^2; 0.0m/s; 0.2m])) .< 1.0e-12)
coeffs

[cstjean]

It's should be doable with another special array type, but this PR only supports units of the form

[a*x a*y a*z
 b*x b*y b*z
 c*x c*y c*z]

There might already be a package that generates arrays of the form your A has.