`.convert` does not need to bind input type with output type

the `.convert()` function needs an input type with a bound constraint on `Coord` and produce an output type with a bound constraint on `Coord`. However, the way it is defined today forces the 2 concrete types to be the same.

```rust
pub fn convert(&self, point: C) -> Result
where
C: Coord,
F: CoordinateType;
```

But it doesn't have to be like this. We could define the function as follows.

```rust
pub fn convert(&self, point: C) -> Result
where
C: Coord,
D: Coord,
F: CoordinateType;
```

Note however that this would be a breaking change for the crate `proj` as now, the expected type should be made explicit and type inference might not always be able to save you.

Start by locating the `.convert()` implementation and its existing Rust tests in the proj crate. Check how the current generic signature is used and whether callers rely on type inference; this is done when differing input and output coordinate types are supported without leaving the documented breaking-change concerns unresolved.

Written by the indexing model from the issue text.