Add method to close file connection

[MattArran]

New method .close() closes any connection opened by e.g. read_box, to the ZarrTiffStore corresponding to the GeoTiff. Fixes #69.

>>> import geotiff
>>> import os
>>> geo_tiff = geotiff.GeoTiff('o41078a7.tif')
>>> geo_tiff.read()[:10,:10]
array([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]], dtype=uint8)
>>> os.remove('o41078a7.tif')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'o41078a7.tif'
>>> geo_tiff.close()
>>> os.remove('o41078a7.tif')
>>>