Handle user-supplied posterior draws

## Usage

```
user_draws(model, draws, ...)
# S3 method for class 'gam'
user_draws(model, draws, index = NULL, ...)
```

## Arguments

- model
a fitted R model. Currently only models fitted by

`mgcv::gam()`

or`mgcv::bam()`

, or return an object that*inherits*from such objects are supported. Here, "inherits" is used in a loose fashion; models fitted by`scam::scam()`

are support even though those models don't strictly inherit from class`"gam"`

as far as`inherits()`

is concerned.- draws
matrix; user supplied posterior draws to be used when

`method = "user"`

.- ...
arguments passed to methods.

- index
a vector to index (subset) the columns of

`draws`

.

## Details

The supplied `draws`

must be a matrix (currently), with 1 column per
model coefficient, and 1 row per posterior draw. The `"gam"`

method has
argument `index`

, which can be used to subset (select) coefficients
(columns) of `draws`

. `index`

can be any valid way of selecting (indexing)
columns of a matrix. `index`

is useful if you have a set of posterior draws
for the entire model (say from `mgcv::gam.mh()`

) and you wish to use those
draws for an individual smooth, via `smooth_samples()`

.