Mark bernoulli_pdf function as unsafe

[kitcatier]

rust-GSL/src/randist/bernoulli.rs

Lines 7 to 9 in 234cfc2

	pub fn bernoulli_pdf(x: u32, p: f64) -> f64 {
	unsafe { sys::gsl_ran_bernoulli_pdf(x, p) }
	}

Hello, it is not a good choice to mark the entire function body as unsafe, which will make the caller ignore the safety requirements that the function parameters must guarantee, the developer who calls the bernoulli_pdf function may not notice this safety requirement.

Marking them unsafe also means that callers must make sure they know what they're doing.

[GuillaumeGomez]

So I have a few questions:

• Why only this function considering there are a lot of others which only contain a FFI function call?
• Did you read gsl_ran_bernoulli_pdf source code before opening this PR? (You can find it here.)
• What's the point of providing a wrapper on top of FFI if we just mark everything as unsafe?