Make lack of implict copy very clear. Add .copy() function.

[nmwsharp]

Right now the main PlyData class cannot be implicitly copied. This is probably the right behavior; doing so is likely a mistake.

However, we can improve the situation in two ways

• Explicitly delete the copy constructor, to make it clear that the behavior is intentional
• Provide a .copy() function for the few situations where one really does want to explicitly copy

[GMudide]

I'm interested

[GMudide]

looks like no active contributors and maintainers ?

[GMudide]

pull request - #37