Allow for custom setting of glm_path in run_glm

[jsta]

It would be useful to allow users to set a custom glm_path rather than relying on the pre-built executables here. Maybe a custom path could be passed to run_glm as an argument and/or read from an environment variable?

The environment variable approach seems less likely to cause errors during typical use. I propose something like the following to be inserted in glm.systemcall:

  if(nchar(Sys.getenv("GLM_PATH")) > 0){
    glm_path <- Sys.getenv("GLM_PATH")
  }