homogenize API for default inputs, dynamic inputs, task inputs, task outputs

This change was prompted by XRD/darfix#92 and a user who tried to do something similar

• set a single value
  • set_default_input
  • set_dynamic_input
• get a single value
  • get_default_input_value
  • get_dynamic_input_value
  • get_task_output_value
  • get_task_input_value
• get variable names
  • get_default_input_names
  • get_dynamic_input_names
  • get_input_names
  • get_output_names
• set multiple values
  • update_default_inputs
  • update_dynamic_inputs
• get multiple values
  • get_default_input_values
  • get_dynamic_input_values
  • get_task_output_values
  • get_task_input_values