add find_apernames function

docs/pysiaf/index.rst

@@ -84,6 +84,21 @@ Frame transformations (``det``, ``sci``, ``idl``, ``tel`` are supported frames):
     # transform from Science frame to Ideal frame
     idl_x, idl_y = nis_cen.sci_to_idl(sci_x, sci_y)
 
+Finding Available Apertures
+***************************
+
+Each Siaf instance has an attribute ``apernames`` giving all available aperture names. There are often many of these::
+
+    siaf = pysiaf.Siaf('NIRSpec')
+    print (len(siaf.apernames))
+    # output: 75
+
+The ``find_apernames`` method allows quick lookup of aperture names matching some substring::
+
+    siaf = pysiaf.Siaf('NIRSpec')
+    siaf.find_apernames('S200')
+    # output: ['NRS_S200A1_SLIT', 'NRS_S200A2_SLIT', 'NRS_S200B1_SLIT']
+
 
 Using sky transforms
 ********************

pysiaf/siaf.py

@@ -522,4 +522,9 @@ def plot_detector_channels(self, frame=None, ax=None):
 
         for ap in self._getFullApertures():
             ap.plot_detector_channels(frame=frame, ax=ax)
-
+
+    def find_apernames(self, substring):
+        """ Return aperture names containing some substring.
+        Simple utility function to search through names of available apertures.
+        """
+        return [name for name in self.apernames if substring in name]