checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/atompaw/package.py

@@ -18,8 +18,8 @@ class Atompaw(Package):
     homepage = "http://users.wfu.edu/natalie/papers/pwpaw/man.html"
     url = "http://users.wfu.edu/natalie/papers/pwpaw/atompaw-4.0.0.13.tar.gz"
 
-    version('4.0.0.13', 'af4a042380356f6780183c4b325aad1d')
-    version('3.1.0.3', 'c996a277e11707887177f47bbb229aa6')
+    version('4.0.0.13', sha256='cbd73f11f3e9cc3ff2e5f3ec87498aeaf439555903d0b95a72f3b0a021902020')
+    version('3.1.0.3', sha256='15fe9a0369bdcc366370a0ecaa67e803ae54534b479ad63c4c7494a04fa3ea78')
 
     depends_on("lapack")
     depends_on("blas")