This function opens strings for output. OpenOString is used to write to a
string with the help of print on. use close to obtain
the string of characters "printed on" the ostream.
Example
/**/ str := OpenOString(); -- open a string for output from CoCoA
/**/ L := [1,2,3]; -- a list
/**/ print L on str; -- print to str
/**/ str;
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/**/ close(str); -- gives a string containing the output sent to str
[1, 2, 3]