Modified use_memmap defaults in SpyFile subclass read methods.

* io/bilfile.py:
  (BilFile.read_subimage): Changed use_memmap default to False.
* io/bipfile.py:
  (BipFile.read_subimage): Changed use_memmap default to False.
* io/bsqfile.py:
  (BsqFile.read_bands): Changed use_memmap default to False.
  (BsqFile.read_subimage): Changed use_memmap default to False.