fix more github-linguist stuff (AMReX-Astro#451)

Author: zingale

.gitattributes

@@ -1,3 +1,12 @@
 *.ipynb linguist-vendored
 *.tex linguist-documentation
+*.bib linguist-documentation
 
+*.[123]d* linguist-vendored
+*.[12] linguist-vendored
+*.mg linguist-vendored
+*.M2 linguist-vendored
+*.3alpha linguist-vendored
+*.nse linguist-vendored
+
+*.H linguist-language=C++