Remove // style comments from source code.

[DennisHeimbigner]

We need to do this periodically because they sneek in.

[t-b]

Does the codebase compile with gcc's -ansi flag? If yes, you could add that to the CI and let gcc do the complaining. See https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html.

[edhartnett]

Good idea. I have added that to my CI system for netCDF.

[WardF]

Same here, I'll add it to the travis CI