Parallelization not supported on Windows

[wleoncio]

Running test-parallel.R on a Windows machine gives the following error:

── Error ('test-parallel.R:78:3'): (code run outside of `test_that()`) ─────────
Error in `makeForkCluster(nnodes = spec, ...)`: fork clusters are not supported on Windows
Backtrace:
    ▆
 1. └─MADMMplasso (local) mad_wrap(legacy = TRUE, parallel = TRUE, pal = FALSE) at test-parallel.R:92:1
 2.   ├─base::suppressMessages(...) at test-parallel.R:78:3
 3.   │ └─base::withCallingHandlers(...)
 4.   └─MADMMplasso::MADMMplasso(...)
 5.     └─parallel::makeCluster(cl1, type = "FORK") at MADMMplasso/R/MADMMplasso.R:210:5
 6.       └─parallel::makeForkCluster(nnodes = spec, ...)

A couple possible solutions

1. Dropping support for parallel = TRUE on Windows
2. Finding an alternative to fork clusters on Windows