ENH: Support multiline header fields in TCK

Treat repeated keys as adding lines to the field

Closes gh-957

Author: effigies

Diff for: nibabel/streamlines/tck.py

@@ -6,6 +6,7 @@
 
 import os
 import warnings
+from contextlib import suppress
 
 import numpy as np
 
@@ -331,6 +332,8 @@ def _read_header(cls, fileobj):
             f.seek(1, os.SEEK_CUR)  # Skip \n
 
             found_end = False
+            key = None
+            tmp_hdr = {}
 
             # Read all key-value pairs contained in the header, stop at EOF
             for n_line, line in enumerate(f, 1):
@@ -343,15 +346,22 @@ def _read_header(cls, fileobj):
                     found_end = True
                     break
 
-                if ':' not in line:  # Invalid header line
+                # Set new key if available, otherwise append to last known key
+                with suppress(ValueError):
+                    key, line = line.split(':', 1)
+                    key = key.strip()
+
+                # Apparent continuation line before any keys are found
+                if key is None:
                     raise HeaderError(f'Invalid header (line {n_line}): {line}')
 
-                key, value = line.split(':', 1)
-                hdr[key.strip()] = value.strip()
+                tmp_hdr.setdefault(key, []).append(line.strip())
 
             if not found_end:
                 raise HeaderError('Missing END in the header.')
 
+            hdr.update({key: '\n'.join(val) for key, val in tmp_hdr.items()})
+
             offset_data = f.tell()
 
             # Set the file position where it was, in case it was previously open