SnapshotTool: compatibility issue with float images

[PierreRaybaut]

The normalization feature should be hidden for float images because it aims at normalizing data to datatype maximum value which has no sense for floating types.

Using this feature on float data leads to the following traceback:

ValueError: Invalid integer data type 'f'.
Traceback (most recent call last):
  File "C:\C2OIQ\python-3.8.10.amd64\lib\site-packages\guiqwt\tools.py", line 912, in end_rect
    self.action_func(plot, QPointF(left, top), QPointF(right, bottom))
  File "C:\C2OIQ\python-3.8.10.amd64\lib\site-packages\guiqwt\tools.py", line 1864, in save_snapshot
    data = io.scale_data_to_dtype(data, dtype=dtype)
  File "C:\C2OIQ\python-3.8.10.amd64\lib\site-packages\guiqwt\io.py", line 44, in scale_data_to_dtype
    info = np.iinfo(dtype)
  File "C:\C2OIQ\python-3.8.10.amd64\lib\site-packages\numpy\core\getlimits.py", line 656, in __init__
    raise ValueError("Invalid integer data type %r." % (self.kind,))
ValueError: Invalid integer data type 'f'.