Reading a raw binary int16 file as uint16

[yagui]

To Reproduce

import numpy as np
from neo.rawio.rawbinarysignalrawio import RawBinarySignalRawIO

data_orig = np.array([0, 32000, -32000], dtype="int16")

with open("data.bin", "wb") as f:
    data_orig.tofile(f)

reader = RawBinarySignalRawIO(filename="data.bin",  dtype="int16", nb_channel=1, signal_gain=1.0)
reader.parse_header()
nb_stream = reader.signal_streams_count()
data_new = reader.get_analogsignal_chunk(block_index=0)
for i in range(data_orig.shape[0]):
    assert data_new[i][0] == data_orig[i], f"{i}: Expected {data_orig[i]}, but got {data_new[i][0]}"