Cleanup custom MPI flow

[TysonRayJones]

Tasks here, samples of new env init here

[TysonRayJones]

Issue: internal consultation of comm_isInit() to check whether QuEST is distributed will need to be changed (e.g. getQuESTEnv().isDistributed, but ideally not an API call like that), so that non-distributed QuEST can be used when the user has MPI initialised for other purposes

[TysonRayJones]

Issue: print functions will check whether they are root before printing - as ergo will an "invalid subcommunicator" style validation error message. The check of rootness will consult the subcommunicator; we will have to fallback to WORLD when communicator is null, so that invalid MPI config error messages are viewable

Edit: fell back to no-MPI-consultation for extra safety

[TysonRayJones]

Opened PR #762