Heterogenous job

#### Feature Request Description

We have jobs that consist of integrating software written for disparate systems. We would like to be able to submit a job that asks for multiple resources of different types. For example, I might need:
- 2 Linux based node with 32GB RAM and 4 cores.
- 1 Windows based node with 8GB RAM and 2 cores.

The whole job should not start until all the requested boxes are available.

#### Describe Preferred Solution

I do not need the UI to be updated to support this as we will be programmatically submitting jobs via the RESTful API. An updated Job XML description that allows for this type of request would be sufficient.

#### Describe Alternatives Considered

We could submit a "master" job that once it begins executing it could submit job(s) to MHPC for the other necessary resources. However, the master would be consuming a slot while waiting for the sub-jobs to queue and get serviced.

#### Additional Context

n/a