fix type instability in balance_statespace

[baggepinnen]

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[JuliaControlBot]

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[codecov]

Codecov Report

Merging #655 (5fc3e4e) into master (08eee7e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #655   +/-   ##
=======================================
  Coverage   86.66%   86.66%           
=======================================
  Files          31       31           
  Lines        3322     3322           
=======================================
  Hits         2879     2879           
  Misses        443      443           

Impacted Files	Coverage Δ
src/types/conversion.jl	97.79% <100.00%> (ø)

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