Functions to check if a smooth is a by-variable one and to test of the type
of by-variable smooth is a factor-smooth or a continous-smooth interaction.

## Usage

```
is_by_smooth(smooth)
is_factor_by_smooth(smooth)
is_continuous_by_smooth(smooth)
by_variable(smooth)
by_level(smooth)
```

## Arguments

- smooth
an object of class `"mgcv.smooth"`