Setup digital communications model configuration

[willu47]

Child of #55

• Define model with inputs, outputs and parameters
• Identify dimensions
• Set up defaults for parameters
• Set up scenarios for inputs
• Generate interventions

[willu47]

@edwardoughton - I think these are the inputs and outputs required for the fixed network model (please edit as necessary):

Inputs:
- name: adoption
  description: 
  dims: []

Outputs:
- name: spend
  description: spend by distribution point
  dims:
  - distribution_points
  - technology
  - year
  - policy
  - spend_type
- name: coverage
  description:
- name: aggregate_coverage
  description:
- name: capacity
  description:

[tomalrussell]

Closed by #82