wr_dat_file: support FLAC output formats.

Writing FLAC formats, like reading FLAC formats, is done using the
soundfile package, which is currently treated as an optional
dependency (since on some systems it requires an external copy of
libsndfile.)

This code performs, in effect, the inverse of _rd_compressed_file.  As
with _rd_compressed_file, this has room to be optimized somewhat.

Author: Benjamin Moody

wfdb/io/_signal.py

@@ -2285,6 +2285,8 @@ def wr_dat_file(
     N/A
 
     """
+    file_path = os.path.join(write_dir, file_name)
+
     # Combine list of arrays into single array
     if expanded:
         n_sig = len(e_d_signal)
@@ -2416,9 +2418,54 @@ def wr_dat_file(
         b_write = b_write.reshape((1, -1))[0]
         # Convert to un_signed 8 bit dtype to write
         b_write = b_write.astype("uint8")
+
+    elif fmt in ("508", "516", "524"):
+        import soundfile
+
+        if any(spf != samps_per_frame[0] for spf in samps_per_frame):
+            raise ValueError(
+                "All channels in a FLAC signal file must have the same "
+                "sampling rate and samples per frame"
+            )
+        if n_sig > 8:
+            raise ValueError(
+                "A single FLAC signal file cannot contain more than 8 channels"
+            )
+
+        d_signal = d_signal.reshape(-1, n_sig, samps_per_frame[0])
+        d_signal = d_signal.transpose(0, 2, 1)
+        d_signal = d_signal.reshape(-1, n_sig)
+
+        if fmt == "508":
+            d_signal = d_signal.astype("int16")
+            np.left_shift(d_signal, 8, out=d_signal)
+            subtype = "PCM_S8"
+        elif fmt == "516":
+            d_signal = d_signal.astype("int16")
+            subtype = "PCM_16"
+        elif fmt == "524":
+            d_signal = d_signal.astype("int32")
+            np.left_shift(d_signal, 8, out=d_signal)
+            subtype = "PCM_24"
+        else:
+            raise ValueError(f"unknown format ({fmt})")
+
+        sf = soundfile.SoundFile(
+            file_path,
+            mode="w",
+            samplerate=96000,
+            channels=n_sig,
+            subtype=subtype,
+            format="FLAC",
+        )
+        with sf:
+            sf.write(d_signal)
+            return
+
     else:
         raise ValueError(
-            "This library currently only supports writing the following formats: 80, 16, 24, 32"
+            "This library currently only supports writing the "
+            "following formats: 80, 16, 24, 32, 508, 516, 524"
         )
 
     # Byte offset in the file
@@ -2433,7 +2480,7 @@ def wr_dat_file(
         b_write = np.append(np.zeros(byte_offset, dtype="uint8"), b_write)
 
     # Write the bytes to the file
-    with open(os.path.join(write_dir, file_name), "wb") as f:
+    with open(file_path, "wb") as f:
         b_write.tofile(f)