Some of .gwy files raise ValueError in io.py.

ValueError: Units 'um' have not been added for .gwy files

# XY, Z units correction
* units for px_to_nm to multiply data array should be picked up from 'si_unit_z' instetad of 'si_unit_xy'.
* Non-square image should be considered. Probably, px_to_nm_z, px_to_nm_x, px_to_nm_y will be necessary.

# Channel index
Data key of gwy sometimes starts from 1 or larger. (ex. '/4/data') It happens when image is cropped on Gwyddion, and it remains after saving even other channels are removed.

A loop to scan indices was added. And it guesses whether if the channel is topography or not by si_unit_z.

Unit is set during scanning of channel index.

# transposed np.ndarray corrected
_gwy_read_component returns transposed dimensions of (xres,yres), this corrected to (yres, xres)

# load_gwy updated
* load_gwy can find multichannel data starting from any index (ex. 4/data)
* load_gwy can guess whether if it is height channel by unitstr
* load_gwy supports m,mm,um for z-unit
* px_to_nm_x, px_to_nm_y were added for future usage, stored in img_dict

io.py modified
 - Conversion from gwy.DataField to np.ndarray was corrected.
   (dimension was transposed)
 - Scale class added to conversion of units and holds conversion factors
   - Initializable by configuration YAML file
   - This object is copied to img_dict of data
      accessible scan.img_dict[filename]["scale"]
      px_to_nm_x, px_to_nm_y values in img_dict from Gwy

Testing and Configurations
 - Resource files (file_landscape.gwy, file.square.gy) added.
 - conftest.py modified to read file.gwy and file_landscape.gwy
 - Test for Scale class
    - test_scale_config was added.
    - test_scale in test_io.py was added.
- "scale" configuration was added to default_config.yaml