Update plot style

[mimischi]

Fixes #96.

Changes made in this pull request:

• Added option to set the plot DPI via --dpi.
• Added option to set the plot font size via --font-size.
• Added option to hide the linear scaling fit via --no-fit.
• Added saner defaults for ylim.
• Added custom xticks and yticks depending on the number of nodes/cores used and the benchmarks' performance.
• xtick steps can be overwritten with the option --xtick-step.
• Added a watermark (text: MDBenchmark) to the top left of the plot. It's added to every plot by default, but can easily be switched off with --no-watermark.

PR Checklist

• Added changelog fragment in ./changelog/ (more information)?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #108 into develop will increase coverage by 0.19%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #108      +/-   ##
===========================================
+ Coverage    96.34%   96.54%   +0.19%     
===========================================
  Files           14       14              
  Lines          684      723      +39     
===========================================
+ Hits           659      698      +39     
  Misses          25       25

Impacted Files	Coverage Δ
mdbenchmark/plot.py	92.7% <100%> (+2.9%)	⬆️

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