Refactoring of PR file structure

Norman Rzepka wants the new functionality as standalone class
He argues by lack of support for remote zip stores

Therefore I revert ReadOnlyZipStore.java to original implementation in zarr-developers/zarr-java d5552e7
I rename class that supports random access Zip files to ReadOnlyFilesystemZipStore
I add test coverage in ReadOnlyFileSystemZipStoreTest

Author: kulvait