`redist_mergesplit_parallel` does not initialize from existing plan if init_plan is NULL.

[christopherkenny]

Current fix for anyone:

Specify init_plan = get_existing(you_map_object)

library(redist)
#> 
#> Attaching package: 'redist'
#> The following object is masked from 'package:stats':
#> 
#>     filter

data(iowa)

ia <- redist_map(iowa, existing_plan = cd_2010, 
                 pop_tol = 0.01)

redist_mergesplit_parallel(ia, 10, 2, ncores = 2)
#> Sampling initial plans with SMCSEQUENTIAL MONTE CARLO
#> Sampling 2 99-unit maps with 4 districts and population between 753973 and 769205.
#> Making split 1 of 3Note: maximum hit; falling back to naive k estimator.
#>  (using k = 1)
#> >------------------------------ 0% | ETA:?
#> ```

> 25% acceptance rate, 100% efficiency.

> Making split 2 of 3Note: maximum hit; falling back to naive k estimator.

> (using k = 1)

> 29% acceptance rate, 99.8% efficiency.

> Making split 3 of 3Note: maximum hit; falling back to naive k estimator.

> (using k = 1)

> 5.4% acceptance rate, 99.6% efficiency.

> A containing 10 sampled plans and 2 reference plans

> Plans have 4 districts from a 99-unit map, and were drawn using Merge-split

> Markov chain Monte Carlo.

> Plans matrix: num [1:99, 1:12] 4 4 1 4 4 1 1 2 1 1 …

> # A tibble: 48 x 4

> draw chain district total_pop

>

> 1 1 1 1 759219

> 2 1 1 2 754687

> 3 1 1 3 764154

> 4 1 1 4 768295

> 5 2 2 1 759219

> 6 2 2 2 754687

> 7 2 2 3 764154

> 8 2 2 4 768295

> 9 1 1 1 756937

> 10 1 1 2 764525

> # … with 38 more rows

get_existing(ia)

> [1] 3 3 1 2 4 1 1 4 1 1 4 4 4 4 3 2 4 4 4 2 4 1 2 4 3 2 2 1 2 4 1 4 1 4 4 3 4 4

> [39] 3 4 4 4 4 2 1 4 4 1 1 2 2 2 1 2 4 2 1 2 2 4 3 2 2 1 3 1 4 2 3 2 4 4 3 4 4 4

> [77] 3 3 1 3 4 2 4 4 4 1 3 3 2 2 3 2 2 4 4 1 4 1 4

```

<sup>Created on 2021-11-20 by the reprex package (v2.0.1)</sup>

The issue appears to be here, where it looks only at init_plan, not init_plans & init_plan, asinit_plan` isn't updated.