reduce allocations in lsim

[baggepinnen]

Benchmark

sys = ssrand(5,5,10, Ts=1);
u = randn(5,10000);
@btime lsim($sys, $u);

# 812.612 μs (11 allocations: 1.98 MiB) # before
# 690.747 μs (7 allocations: 1.22 MiB)  # after

[codecov]

Codecov Report

Merging #651 (c636fb3) into master (4feb58d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #651      +/-   ##
==========================================
+ Coverage   86.62%   86.63%   +0.01%     
==========================================
  Files          31       31              
  Lines        3311     3315       +4     
==========================================
+ Hits         2868     2872       +4     
  Misses        443      443              

Impacted Files	Coverage Δ
src/timeresp.jl	90.00% <100.00%> (+0.31%)	⬆️

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