Prevent memoizing from failing if the lwfDecoder jar is not present

loci_tools.jar and bioformats_package.jar do not contain the lwfDecoder
class (so that users more easily use the non-stub jar if they have it).
This caused memo file creation to fail when
loci_tools.jar/bioformats_package.jar were the sole classpath entries.
Zip: do not store the helper reader in the memo file
Merge remote-tracking branch 'melissa/memoizing-fixes' into memo-cleanup