Add license headers.

pymbolic/__init__.py

pymbolic/algorithm.py

pymbolic/compiler.py

pymbolic/cse.py

pymbolic/functions.py

pymbolic/mapper/__init__.py

pymbolic/mapper/c_code.py

pymbolic/mapper/collector.py

pymbolic/mapper/constant_folder.py

pymbolic/mapper/dependency.py

pymbolic/mapper/differentiator.py

pymbolic/mapper/evaluator.py

pymbolic/mapper/expander.py

pymbolic/mapper/flattener.py

pymbolic/mapper/flop_counter.py

pymbolic/mapper/stringifier.py

pymbolic/mapper/substitutor.py

pymbolic/mapper/unifier.py

pymbolic/maxima.py

pymbolic/parser.py