Hotfix for Windows OS support

This MR provides a hotfix for #120 by making import fcntl optional in Util.py, so that code which relies on Util.py can load successfully on Windows.

With this fix applied, if a Windows user tries to call util.write_state_file or util.read_state_file it will fail with a warning without blocking further execution. This should only affect Windows users that run MiniSeedFactory with convert_channels ~= None (unlikely!). Even if they do try this it should run successfully but without any reading or writing of state files.

I tested this fix on a Windows installation of Python and appears to work.