typestability and compile-time improvements

[baggepinnen]

some simple type-stability improvements takes

julia> @time subspaceid(d, 2);
 13.317923 seconds (48.73 M allocations: 2.806 GiB, 8.91% gc time, 99.96% compilation time)

to

 9.236199 seconds (27.19 M allocations: 1.325 GiB, 12.12% gc time, 99.98% compilation time)

  julia> @time tfest(d);
  7.411997 seconds (25.10 M allocations: 1.631 GiB, 8.98% gc time, 99.96% compilation time) # before
  3.793959 seconds (14.25 M allocations: 944.387 MiB, 8.01% gc time, 99.93% compilation time) # after

[codecov[bot]]

Codecov Report

Merging #89 (0e85812) into master (4614ed9) will decrease coverage by 0.14%. The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   83.79%   83.64%   -0.15%     
==========================================
  Files          12       12              
  Lines        2005     2006       +1     
==========================================
- Hits         1680     1678       -2     
- Misses        325      328       +3     

Flag	Coverage Δ
unittests	83.64% <87.50%> (-0.15%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/utils.jl	63.26% <0.00%> (-7.33%)	:arrow_down:
src/types.jl	70.86% <66.66%> (ø)
src/subspace2.jl	88.37% <80.00%> (+0.03%)	:arrow_up:
src/arx.jl	86.93% <95.45%> (+0.18%)	:arrow_up:
src/ControlSystemIdentification.jl	76.56% <100.00%> (ø)
src/frd.jl	85.34% <100.00%> (ø)
src/frequency_weights.jl	40.62% <100.00%> (-8.03%)	:arrow_down:
src/plotting.jl	92.70% <100.00%> (+0.06%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 4614ed9...0e85812. Read the comment docs.