[cpp] add 'const' to constant functions

[MSchleinkoferV]

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Some functions of class ParseTree, e. g.

• std::string ParseTree::toStringTree(…)
• std::string ParseTree::toString()
• std::string ParseTree::getText()

are not intended to modify the object. Thus they should also be tagged as const in c++. Otherwise when handling const objects, these functions are not available. Actually one can not restrict objects as const.

Note: ParseTreeType ParseTree::getTreeType() const is already const