Access environment variables from MaBL spec

[CThuleHansen]

Currently the FMUs are defined in both the environment.json and the MaBL spec. For example:

"fmus": {
    "{x1}": "target/test-classes/watertankcontroller-c.fmu",
    "{x2}": "target/test-classes/singlewatertank-20sim.fmu"
  },

and

FMI2 tankcontroller = load("FMI2", "{8c4e810f-3df3-4a00-8276-176fa3c9f000}", "target/test-classes/watertankcontroller-c.fmu");
FMI2 SingleWatertank = load("FMI2", "{cfc65592-9ece-4563-9705-1581b6e7071c}",  "target/test-classes/watertankcontroller-c.fmu");

To avoid duplication it should be possible to access the environment in the MaBL spec. For example:

FMI2 tankcontroller = load("FMI2", "{8c4e810f-3df3-4a00-8276-176fa3c9f000}", $env.fmus("{x1})"$);
FMI2 SingleWatertank = load("FMI2", "{cfc65592-9ece-4563-9705-1581b6e7071c}",  $env.fmus("{x2})"$);