Convenient matcher on f32/f64 with error rate

In [this patch](https://github.com/apache/datasketches-rust/pull/75/files#diff-ca48ad422571f43d769560d89d2df4d4d7c9e831353d679b9c81eaf244d2b894) we have a test case like:

```rust
const RELATIVE_ERROR_FOR_LG_K_11: f64 = 0.02;
const N: usize = 10000;
const N_F64: f64 = N as f64;
assert_that!(
sketch.estimate(),
near(N_F64, RELATIVE_ERROR_FOR_LG_K_11 * N_F64)
);
```

It would be better to have a matcher like `assert_that!(sketch.estimate(), near_(N_F64, RELATIVE_ERROR_FOR_LG_K_11)` that expresses:

```
abs(sketch.estimate() - N_F64) <= N_F64 * RELATIVE_ERROR_FOR_LG_K_11
```

Not sure if there is already good name and implementation in other language's matcher libs.