more spectral extraction improvements for downstream use (#3227)

* only show function dropdown if multiple choices
* improve styling of photutils docs link when plugin is wide
* don't show warning if spectrum_y_units is not even defined

jdaviz/configs/cubeviz/plugins/spectral_extraction/spectral_extraction.vue

@@ -209,16 +209,17 @@
           api_hint="plg.aperture_method ="
           :api_hints_enabled="api_hints_enabled"
           :hint="'Extract '+resulting_product_name+' using an aperture masking method in place of the subset mask.'"
-        >
+        />
+        <v-row>
           <j-docs-link>
-            See the <j-external-link link='https://photutils.readthedocs.io/en/stable/aperture.html#aperture-and-pixel-overlap'
-            linktext='photutils docs'></j-external-link>
-            for more details on aperture masking methods.
+              See the <j-external-link link='https://photutils.readthedocs.io/en/stable/aperture.html#aperture-and-pixel-overlap'
+              linktext='photutils docs'></j-external-link> for more details on aperture masking methods.
           </j-docs-link>
-        </plugin-select>
+        </v-row>
       </div>
 
       <plugin-select
+        v-if="function_items.length > 1"
         :items="function_items.map(i => i.label)"
         :selected.sync="function_selected"
         label="Function"
@@ -258,7 +259,7 @@
         @click:action="spectral_extraction"
       >
         <v-alert
-          v-if="results_units !== spectrum_y_units"
+          v-if="spectrum_y_units && results_units !== spectrum_y_units"
           type='warning'
           style="margin-left: -12px; margin-right: -12px"
         >