`create_index` should be multithreaded by default

[brycefrank]

pyfor/pyfor/collection.py

Lines 226 to 232 in 091a666

	def create_index(self):
	"""
	For each file in the collection, creates `.lax` files for spatial indexing using the default values.
	"""
	for las_path in self['las_path']:
	laxpy.file.init_lax(las_path)