pyfastnoisesimd dependency

[benjaminthedonkey]

Thanks for sharing this amazing project. It's truly a gem. I just want to mention, the dependency with pyfastnoisesimd might be a real hinder. On Windows most people will have to install 800+ MB (MSVC2015 Build Tools) and countless headaches to get this working ( a wheel is only provided for python 3.6 ). I'm not an expert, but seems like this library is only required to use one of the flowfields functions ? Please consider making this dependency optional, if it's not installed, the app should run. thanks.