add "only water" test on San Francisco image

[namsellem]

This test has been implemented to verify proper usage of skimage.measure.label() function.
Previous implementation didn't fit with only water tiled watermask detection.

Tests

Name                                        Stmts   Miss  Cover
---------------------------------------------------------------
slurp/__init__.py                               7      2    71%
slurp/eomultiprocessing/__init__.py             0      0   100%
slurp/eomultiprocessing/slurp_executor.py     268    186    31%
slurp/eomultiprocessing/slurp_manager.py       47     16    66%
slurp/eomultiprocessing/utils.py               52     17    67%
slurp/masks/__init__.py                         7      0   100%
slurp/masks/shadowmask.py                     137      7    95%
slurp/masks/stack_masks.py                    223     13    94%
slurp/masks/urbanmask.py                      265     30    89%
slurp/masks/vegetationmask.py                 399     22    94%
slurp/masks/watermask.py                      358     30    92%
slurp/post_process/__init__.py                  0      0   100%
slurp/post_process/morphology.py               32      4    88%
slurp/prepare/__init__.py                       7      0   100%
slurp/prepare/analyse_glcm.py                  73      9    88%
slurp/prepare/aux_files.py                      7      0   100%
slurp/prepare/geometry.py                     110      2    98%
slurp/prepare/prepare.py                      227     28    88%
slurp/prepare/primitives.py                    29      0   100%
slurp/prepare/validity.py                       7      1    86%
slurp/tools/__init__.py                         7      0   100%
slurp/tools/constant.py                         7      0   100%
slurp/tools/io_utils.py                        30      7    77%
slurp/tools/misc.py                            26     26     0%
slurp/tools/mylogger.py                        27     14    48%
slurp/tools/profile_utils.py                   78     14    82%
slurp/tools/pydantic_class.py                 145      0   100%
slurp/tools/random_forest_utils.py             23      0   100%
slurp/tools/utils.py                           56      5    91%
---------------------------------------------------------------
TOTAL                                        2654    433    84%

=== 44 passed, 23 deselected, 108 warnings in 487.58s (0:08:07) ===