Fix ResourceWarning from unclosed detached data NRRD files (#90)

Enable all warnings during tests to see additional problems in the code. The following warning appeared:
```
ResourceWarning: unclosed file <_io.BufferedReader name='<PYNRRD_DIR>\\nrrd\\tests\\data\\BallBinary30x30x30.raw'>
  tb.tb_frame.clear()
```

Problem resolved by closing the file handle before raising errors. The only time this issue occurs is when the data is detached from the header. The detached data filename is loaded using an `open` call without a "with" block.

nrrd/reader.py

@@ -356,6 +356,7 @@ def read_data(header, fh=None, filename=None):
             data_filename = os.path.join(os.path.dirname(filename), data_filename)
 
         # Override the fh parameter with the data filename
+        # Note that this is opened without a "with" block, thus it must be closed manually in all circumstances
         fh = open(data_filename, 'rb')
 
     # Get the total number of data points by multiplying the size of each dimension together
@@ -368,10 +369,18 @@ def read_data(header, fh=None, filename=None):
         for _ in range(line_skip):
             fh.readline()
     else:
+        # Must close the file because if the file was opened above from detached filename, there is no "with" block to
+        # close it for us
+        fh.close()
+
         raise NRRDError('Invalid lineskip, allowed values are greater than or equal to 0')
 
     # Skip the requested number of bytes or seek backward, and then parse the data using NumPy
     if byte_skip < -1:
+        # Must close the file because if the file was opened above from detached filename, there is no "with" block to
+        # close it for us
+        fh.close()
+
         raise NRRDError('Invalid byteskip, allowed values are greater than or equal to -1')
     elif byte_skip >= 0:
         fh.seek(byte_skip, os.SEEK_CUR)
@@ -394,6 +403,10 @@ def read_data(header, fh=None, filename=None):
         elif header['encoding'] in ['bzip2', 'bz2']:
             decompobj = bz2.BZ2Decompressor()
         else:
+            # Must close the file because if the file was opened above from detached filename, there is no "with" block
+            # to close it for us
+            fh.close()
+
             raise NRRDError('Unsupported encoding: "%s"' % header['encoding'])
 
         # Loop through the file and read a chunk at a time (see _READ_CHUNKSIZE why it is read in chunks)
@@ -412,8 +425,7 @@ def read_data(header, fh=None, filename=None):
         # NumPy
         data = np.fromstring(decompressed_data[byte_skip:], dtype)
 
-    # Close the file
-    # Even if opened using with keyword, closing it does not hurt
+    # Close the file, even if opened using "with" block, closing it manually does not hurt
     fh.close()
 
     if total_data_points != data.size:
@@ -457,4 +469,4 @@ def read(filename, custom_field_map=None):
         header = read_header(fh, custom_field_map)
         data = read_data(header, fh, filename)
 
-        return data, header
+    return data, header

nrrd/tests/util.py

@@ -1,5 +1,9 @@
 import os
 import unittest
+import warnings
+
+# Enable all warnings
+warnings.simplefilter("always")
 
 DATA_DIR_PATH = os.path.join(os.path.dirname(__file__), 'data')
 RAW_NRRD_FILE_PATH = os.path.join(DATA_DIR_PATH, 'BallBinary30x30x30.nrrd')