Added use_memmap argument to all read_* methods of SpyFile subclasses.

This arg indicates whether to use the memmap interface or to do a direct
read of the file's data. The arg currently defaults to True in all cases
but the default will be modified for each subclass-method to optimize
I/O performance.
Updated doc strings.
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.
Merge branch 'read_memmap_defaults' [closes #9].