fix fft freq primitive to return np.array and add json file of the primitive

Author: SaraPido

sigpro/primitives/sigpro/transformations/frequency/fftfreq/fft_freq.json

@@ -0,0 +1,30 @@
+{
+    "name": "sigpro.transformations.frequency.fftfreq.fft_freq",
+    "primitive": "sigpro.transformations.frequency.fftfreq.fft_freq",
+    "classifiers": {
+        "type": "transformation",
+        "subtype": "frequency"
+    },
+    "produce": {
+        "args": [
+            {
+                "name": "amplitude_values",
+                "type": "numpy.ndarray"
+            },
+            {
+                "name": "sampling_frequency",
+                "type": "float"
+            }
+        ],
+        "output": [
+            {
+                "name": "amplitude_values",
+                "type": "numpy.ndarray"
+            },
+            {
+                "name": "frequency_values",
+                "type": "numpy.ndarray"
+            }
+        ]
+    }
+}

sigpro/transformations/frequency/fftfreq.py

@@ -6,15 +6,15 @@ def fft_freq(amplitude_values, sampling_frequency):
 
     Args:
         amplitude_values (np.ndarray):
-            A numpy array with the signal values.
+            A numpy array with the fft values.
         sampling_frequency (int or float):
             Sampling frequency value passed in Hz.
     Returns:
         tuple:
             * `amplitude_values (numpy.ndarray)`
             * `frequency_values (numpy.ndarray)`
     """
-    amplitude_values = amplitude_values
+    amplitude_values = np.array(amplitude_values)
     frequency_values = np.fft.fftfreq(len(amplitude_values), 1 / sampling_frequency)
 
     return amplitude_values, frequency_values