Add a constant to estimated values

## Usage

```
add_constant(object, constant = NULL, ...)
# S3 method for smooth_estimates
add_constant(object, constant = NULL, ...)
# S3 method for smooth_samples
add_constant(object, constant = NULL, ...)
# S3 method for mgcv_smooth
add_constant(object, constant = NULL, ...)
# S3 method for parametric_effects
add_constant(object, constant = NULL, ...)
# S3 method for tbl_df
add_constant(object, constant = NULL, column = NULL, ...)
# S3 method for evaluated_parametric_term
add_constant(object, constant = NULL, ...)
```