cf.CONSTANTS
============

.. data:: cf.CONSTANTS

   A dictionary of useful constants.

   Whilst the dictionary may be modified directly, it is safer to
   retrieve and set the values with a function where one is
   provided. This is due to interdependencies between some values.

   :Keys:

        ATOL : float
	    The value of absolute tolerance for testing numerically
	    tolerant equality. Retrieved and set with :func:`.ATOL`.

        CHUNKSIZE : int
	    The chunk size (in bytes) for data storage and
	    processing. Retrieved and set with :func:`.CHUNKSIZE`.

        FM_THRESHOLD : float
	    The minimum amount of memory (in kibibytes) to be kept
	    free for temporary work space. Retrieved and set with
	    :func:`.FM_THRESHOLD`.

        MINNCFM : int
	    The number of chunk sizes to be kept free for temporary
	    work space.

        RTOL : float
	    The value of relative tolerance for testing numerically
	    tolerant equality. Retrieved and set with :func:`.RTOL`.

        TEMPDIR : str
	    The location to store temporary files. By default, use the
	    default directory used by the :mod:`tempfile` module.

   **Examples**

   >>> cf.CONSTANTS
   {'ATOL': 2.2204460492503131e-16,
    'CHUNKSIZE': 104857600,
    'FM_THRESHOLD': 1024000.0,
    'MINNCFM': 10,
    'RTOL': 2.2204460492503131e-16,
    'TEMPDIR': '/tmp'}
   >>> cf.CONSTANTS['TEMPDIR'] = '/home/me/tmp'
   >>> cf.CONSTANTS['MINNCFM'] = 15
   >>> cf.CHUNKSIZE(2**30)
   >>> cf.CONSTANTS['CHUNKSIZE']
   1073741824
   >>> cf.CONSTANTS['FM_THRESHOLD']
   15728640.0