Fix BP3Reader so that you can open for read a file with the same name…

… you used for opening for write.

This addresses Issue #1499 (closed). It doesn't change current behavior where the current behavior is successful. However, where current behavior would fail because a file isn't found, this tries again, adding the .bp suffix. This allows using the same string in reader-side open as was used in writer-side open. There might be an easier way to do this, but this was the least messy way I could come up with.