Macro linspace 

macro_rules! linspace {
    ($dtype:ty, $a:expr, $b:expr, $n:expr) => { ... };
}

Create an array of n equidistant points with first value a and second value b.

The invocation let arr = linspace!(dtype, a, b, n) creates an array of n equidistant points with start value a and final value b. dtype can be either f32 or f64.