Speedup visspec assembly / fix deprecation warning

[dweindl]

• Don't use deprecated pandas.DataFrame.append
• Concatenate all at once

[codecov-commenter]

Codecov Report

Merging #135 (5892a80) into develop (3a0a218) will increase coverage by 0.01%.
The diff coverage is 84.61%.

@@             Coverage Diff             @@
##           develop     #135      +/-   ##
===========================================
+ Coverage    77.28%   77.30%   +0.01%     
===========================================
  Files           27       27              
  Lines         2959     2956       -3     
  Branches       724      724              
===========================================
- Hits          2287     2285       -2     
+ Misses         489      488       -1     
  Partials       183      183              

Impacted Files	Coverage Δ
petab/visualize/plotting.py	86.00% <84.61%> (+0.19%)	⬆️

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