fix shebangs in R scripts and update insertheader() to preserve them
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shebangs https://en.wikipedia.org/wiki/Shebang_(Unix) are used to inform the shell about the interpreter to be used to run a script, and they have to be on the first line of the file
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by putting the header always in the first line, insertheader() destroyed the shebangs of certain R scripts, reducing usability
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insertheader() now replaces existing headers at their current position, while adding new ones to the first line