Fix bounds errors in default x_abschange and x_relchange

[devmotion]

I noticed that currently the default x_abschange and x_relchange assume that the state is non-empty when running parameter estimation tests with an artificial example without any parameters. Clearly, that case is not useful in practice but nevertheless I think it would be good to fix the current unsafe behaviour and use the opportunity to simplify the existing code.

One consideration in this PR is whether to use StatsBase.Linfdist or whether only to copy this arguably very simple function. Given that StatsBase is not needed in this package otherwise (see #1221), I opted for copying and adapting the implementation.

[github-actions]

Benchmark Results (Julia vlts)

Time benchmarks

	master	8b91563...	master / 8b91563...
multivariate/solvers/first_order/AdaMax	0.543 ± 0.009 ms	0.542 ± 0.009 ms	1 ± 0.023
multivariate/solvers/first_order/Adam	0.541 ± 0.0092 ms	0.54 ± 0.0092 ms	1 ± 0.024
multivariate/solvers/first_order/BFGS	0.262 ± 0.0081 ms	0.261 ± 0.0084 ms	1.01 ± 0.045
multivariate/solvers/first_order/ConjugateGradient	0.18 ± 0.003 ms	0.181 ± 0.0029 ms	0.999 ± 0.023
multivariate/solvers/first_order/GradientDescent	1.54 ± 0.012 ms	1.54 ± 0.011 ms	1 ± 0.01
multivariate/solvers/first_order/LBFGS	0.232 ± 0.0078 ms	0.232 ± 0.0073 ms	0.997 ± 0.046
multivariate/solvers/first_order/MomentumGradientDescent	2.17 ± 0.014 ms	2.17 ± 0.012 ms	1 ± 0.0085
multivariate/solvers/first_order/NGMRES	0.425 ± 0.01 ms	0.426 ± 0.01 ms	0.999 ± 0.034
time_to_load	0.407 ± 0.0055 s	0.407 ± 0.0039 s	1 ± 0.017

Memory benchmarks

	master	8b91563...	master / 8b91563...
multivariate/solvers/first_order/AdaMax	0.34 k allocs: 7.19 kB	0.34 k allocs: 7.19 kB	1
multivariate/solvers/first_order/Adam	0.34 k allocs: 7.19 kB	0.34 k allocs: 7.19 kB	1
multivariate/solvers/first_order/BFGS	0.336 k allocs: 15 kB	0.336 k allocs: 15 kB	1
multivariate/solvers/first_order/ConjugateGradient	0.338 k allocs: 13.6 kB	0.338 k allocs: 13.6 kB	1
multivariate/solvers/first_order/GradientDescent	1.89 k allocs: 0.0713 MB	1.89 k allocs: 0.0713 MB	1
multivariate/solvers/first_order/LBFGS	0.317 k allocs: 14.2 kB	0.317 k allocs: 14.2 kB	1
multivariate/solvers/first_order/MomentumGradientDescent	2.24 k allocs: 0.077 MB	2.24 k allocs: 0.077 MB	1
multivariate/solvers/first_order/NGMRES	1.41 k allocs: 0.112 MB	1.41 k allocs: 0.112 MB	1
time_to_load	0.153 k allocs: 14.5 kB	0.153 k allocs: 14.5 kB	1

A plot of the benchmark results has been uploaded as an artifact at .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.83%. Comparing base (432fc63) to head (8b91563).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1222      +/-   ##
==========================================
+ Coverage   86.77%   86.83%   +0.06%     
==========================================
  Files          45       44       -1     
  Lines        3539     3540       +1     
==========================================
+ Hits         3071     3074       +3     
+ Misses        468      466       -2     

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