Gets the parameter associated with the OpAtom at the specified
index and validates that the atom is of the correct type.
Only the top level atoms are checked. So for example, the TJ
operator takes an ArrayAtom consisting of NumAtoms and StringAtoms.
This function will ensure that there is one ArrayAtom available as
a parameter. However it will not ensure that all items inside the
array are NumAtoms or StringAtoms.
If the atom is not of the correct type, is not available in the
array (e.g. because the array is too small) or the operator is
unrecognized then the return value will be null. If the operator
does not take any parameters the return will be null.
The Atom is the actual object in the supplied array so changing
the value of the Atom will change the value in the array. If the
index does not point to an OpAtom or is not inside the array bounds
then an exception will be thrown.
This function supports all the standard PDF operator types as
defined in the PDF Specification apart from BI, ID and EI which are
used for inline images. The reason these are not supported is
because this sequence is read as a DictAtom with embedded data so
you should never encounter it.