Added num2sig() which converts p-value to significance (see #8)

dokdo/api2/num2sig.py

@@ -0,0 +1,34 @@
+def num2sig(num):
+    """Convert a p-avalue to a signifiacne annotation.
+
+    Significance lebels are defined as follows:
+
+    | P-value              | Signifiacne |
+    | -------------------- | ----------- |
+    |   0.05 < P           | ns          |
+    |   0.01 < P <= 0.05   | *           |
+    |  0.001 < P <= 0.01   | **          |
+    | 0.0001 < P <= 0.001  | ***         |
+    |          P <= 0.0001 | ****        |
+
+    Parameters
+    ----------
+    num : float
+        P-value to be converted.
+
+    Returns
+    -------
+    str
+        Signifiance annotation.
+    """
+    if 0.05 < num:
+        sig = 'ns'
+    elif 0.01 < num <= 0.05:
+        sig = '*'
+    elif 0.001 < num <= 0.01:
+        sig = '**'
+    elif 0.0001 < num <= 0.001:
+        sig = '***'
+    else:
+        sig = '****'
+    return sig