Add skip_add param to StaticAddToTrait trait methods

[mvorisek]

Important for usages like:

public function add($seed) {
    return self::addToWithClassName($this, $seed);
}

as without skip_param the add() method is called again from addToWithClassName() method which can be unexpected when the add() method is extended and the add() method from the top-most scope is called ($parent->add()).

The skip_add param allows to implemented it that the add() method from the example is never called twice as the addToWithClassName method can be used as a factory only then.

[codecov]

Codecov Report

Merging #143 into develop will increase coverage by 0.38%.
The diff coverage is 81.81%.

@@              Coverage Diff              @@
##             develop     #143      +/-   ##
=============================================
+ Coverage      97.84%   98.22%   +0.38%     
+ Complexity       436      434       -2     
=============================================
  Files             24       24              
  Lines           1020     1015       -5     
=============================================
- Hits             998      997       -1     
+ Misses            22       18       -4     

Impacted Files	Coverage Δ	Complexity Δ
src/StaticAddToTrait.php	76.31% <81.81%> (+6.54%)	16.00 <16.00> (-2.00)	⬆️

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