”scalar“ is a bad name for a variable or parameter

Describe the role, not the type! e.g.

```swift
/// Returns Jacobians that scale each element by `scalar`.
func jacobians(scalar: Double) -> AnyGaussianFactorArrayBuffer {
```

should be:

```swift
/// Returns Jacobians that scale each element by `scaleFactor`.
func jacobians(scaleFactor: Double) -> AnyGaussianFactorArrayBuffer {
```