align IVM parsing to other plugin parsing

[tylerperyea]

The parsing of plugins/extensions in GSRS largely follows the same pattern:

extension:{
  releventClass:<class_name>,
  //optional parameters
  parameters:{
      parameter1:value1,
      ...
  }
}

We usually use "parameters" but also allow for the same variables to be put in the main config object instead, like:

extension:{
  releventClass:<class_name>,
  //optional parameters
  parameter1:value1
}

This is done like this for scheduledTasks, import functions, export functions, validators and a few other extensions. IVMs are one of the few that doesn't support the alternative representation. Here, this PR allows it to work like the others.