patient_birth_date dtype should be signed integer in nibabel.ecat

Hi, I noticed that in [`nibabel/ecat.py`](https://github.com/nipy/nibabel/blob/master/nibabel/ecat.py), the `patient_birth_date` field is defined as: ```python ('patient_birth_date', np.uint32), ``` This causes incorrect date interpretations, for example producing birth dates in the future rather than before the Unix epoch. It seems this field should be a **signed integer** (`np.int32`) to correctly handle dates. At least in the ECAT files I have, `patient_birth_date` **must be signed integers** to interpret dates correctly. Tools like Vinci read these files correctly when treated as signed. **Example:** ```python import numpy as np import datetime, calendar # 19 Dec 1915 at midnight UTC ts = calendar.timegm(datetime.datetime(1915, 12, 19).timetuple()) # Structured array with wrong dtype dtype_wrong = [('patient_birth_date', np.uint32)] arr_wrong = np.array([(ts,)], dtype=dtype_wrong) # Structured array with correct dtype dtype_right = [('patient_birth_date', np.int32)] arr_right = np.array([(ts,)], dtype=dtype_right) # Convert back to datetime dt_wrong = datetime.datetime.utcfromtimestamp(arr_wrong['patient_birth_date'][0]) dt_right = datetime.datetime.utcfromtimestamp(arr_right['patient_birth_date'][0]) print("Unsigned (wrong):", arr_wrong['patient_birth_date'][0], "->", dt_wrong) print("Signed (correct):", arr_right['patient_birth_date'][0], "->", dt_right) ``` **Example output:** ``` Unsigned (wrong): 2589690496 -> 2052-01-24 06:28:16 Signed (correct): -1705276800 -> 1915-12-19 00:00:00 ``` Changing it to `np.int32` fixes this issue. I can also prepare a **PR** for this update if that’s helpful. Thanks!