Use a single Array in Transform3D, parameterize Transform3D on array type

[tkoolen]

Towards #212, #207.

Other changes required by this move:

• add rotation and translation functions to replace accessing members of Transform3D
• change signatures of eye and rand methods.

A bit faster on 0.6, as expected.

[codecov]

Codecov Report

Merging #215 into master will decrease coverage by 0.01%.
The diff coverage is 96%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   94.85%   94.83%   -0.02%     
==========================================
  Files          18       18              
  Lines        2041     2053      +12     
==========================================
+ Hits         1936     1947      +11     
- Misses        105      106       +1

Impacted Files	Coverage Δ
src/joint.jl	93.75% <ø> (ø)	⬆️
src/mechanism.jl	90.32% <ø> (ø)	⬆️
src/joint_types.jl	94.28% <100%> (-0.07%)	⬇️
src/mechanism_state.jl	98.1% <100%> (ø)	⬆️
src/rigid_body.jl	90.9% <100%> (ø)	⬆️
src/parse_urdf.jl	93.67% <100%> (ø)	⬆️
src/mechanism_manipulation.jl	98.07% <100%> (ø)	⬆️
src/frames.jl	92.22% <94.44%> (+1.19%)	⬆️
src/spatial.jl	92.95% <95.23%> (-0.31%)	⬇️

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