Harmonize Pigosat's finalizer with Go convention

[wkschwartz]

*os.File use a finalizer to close the file. However, it also makes the Close method publicly available so you can close the file yourself. Additionally, it clears the finalizer Close is called.

• Make Pigosat.delete publicly available
• Acquire write lock in Pigosat.delete
• Close OutputFile #17
• Clear finalizer after delete finishes