improve api, cleanup and many other things

[aldas]

No description provided.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@729b684). Click here to learn what that means.
The diff coverage is 62.2%.

@@            Coverage Diff            @@
##             master       #1   +/-   ##
=========================================
  Coverage          ?   69.89%           
  Complexity        ?      131           
=========================================
  Files             ?        8           
  Lines             ?      382           
  Branches          ?        0           
=========================================
  Hits              ?      267           
  Misses            ?      115           
  Partials          ?        0

Impacted Files	Coverage Δ	Complexity Δ
src/RrdRowValue.php	16.12% <16.12%> (ø)	9 <9> (?)
src/Rra/RraInfo.php	55% <52.94%> (ø)	6 <3> (?)
src/RrdReader.php	65.07% <68.18%> (ø)	26 <20> (?)
src/RrdDs.php	67.64% <70%> (ø)	8 <4> (?)
src/Rra/Rra.php	71.42% <73.68%> (ø)	16 <9> (?)
src/RrdFile.php	90.47% <90%> (ø)	15 <7> (?)
src/RrdHeader.php	82.5% <90.9%> (ø)	37 <4> (?)

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