Rename configure.in to configure.ac (#13)

* Initial commit

* Rename configure.in to configure.ac

This fixes the following warning:

```
$ autoreconf -i
autoreconf: warning: autoconf input should be named 'configure.ac', not 'configure.in'
aclocal: warning: autoconf input should be named 'configure.ac', not 'configure.in'
```

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Co-authored-by: Bryan Irby <bryan.k.irby@nasa.gov>