New model name feature and bug fix

List of changes:

• Add a new model_name parameter to core.Model (#94)
  • The default model remains the same ("fasterrcnn_resnet50_fpn", or Model.DEFAULT)
  • New options include the following:
    • "fasterrcnn_mobilenet_v3_large_fpn" (Model.MOBILENET)
    • "fasterrcnn_mobilenet_v3_large_320_fpn" (Model.MOBILENET_320)
• Fix occasional bug with drawing bounding boxes for the detect_live function (#101)