ENH: Transition SourceTarball.bash script for .cid

We fetch from a local IPFS gateway to ensure the data can be obtained
via IPFS. This also ensures the script will run offline, and it runs
much faster. The IPFS gateway will verify CIDs; we do not need to verify
them separately.

Re: #3760

Documentation/Maintenance/Release.md

@@ -358,6 +358,9 @@ endings.
 
 The `InsightData` tarballs are generated along with the source code tarballs.
 
+Data is fetched from [IPFS]. An IPFS daemon must be running to fetch the data
+- [ipfs-desktop] is recommended.
+
 Once the repository has been tagged, we use the following script in the
 repository to create the tarballs:
 
@@ -873,6 +876,8 @@ excellent packaging.
 [documentation page]: https://www.itk.org/ITK/help/documentation.html
 [download page]: https://itk.org/ITK/resources/software.html
 [GitHub]: https://github.com/InsightSoftwareConsortium/ITK
+[IPFS]: https://ipfs.tech/
+[ipfs-desktop]: https://github.com/ipfs/ipfs-desktop/releases
 [ITKPythonPackage]: https://itkpythonpackage.readthedocs.io/en/latest/index.html
 [ITK discussion]: https://discourse.itk.org/
 [Image.sc Forum]: https://image.sc

Utilities/Maintenance/SourceTarball.bash

@@ -36,27 +36,20 @@ return_pipe_status() {
 
 find_data_objects() {
   git ls-tree --full-tree -r "$1" |
-  egrep '\.(md5)$' |
+  egrep '\.(cid)$' |
   while read mode type obj path; do
     case "$path" in
-      *.md5)  echo MD5/$(git cat-file blob $obj) ;;
+      *.cid)  echo CID/$(git cat-file blob $obj) ;;
       *)      die "Unknown ExternalData content link: $path" ;;
     esac
   done | sort | uniq
   return_pipe_status
 }
 
-validate_MD5() {
-  md5sum=$(md5sum "$1" | sed 's/ .*//') &&
-  if test "$md5sum" != "$2"; then
-    die "Object MD5/$2 is corrupt: $1"
-  fi
-}
-
 download_object() {
   algo="$1" ; hash="$2" ; path="$3"
   mkdir -p $(dirname "$path") &&
-  if curl -L "https://www.itk.org/files/ExternalData/$algo/$hash" -o "$path.tmp$$" 1>&2; then
+  if curl -L "http://127.0.01:8080/ipfs/$hash" -o "$path.tmp$$" 1>&2; then
     mv "$path.tmp$$" "$path"
   else
     rm -f "$path.tmp$$"
@@ -78,7 +71,6 @@ index_data_objects() {
       download_object "$algo" "$hash" "$path" &&
       file="$path"
     fi &&
-    validate_$algo "$file" "$hash" &&
     obj=$(git hash-object -t blob -w "$file") &&
     echo "100644 blob $obj	$path" ||
     return