Mackey-Glass task specification

[jasonlyik]

Update the mackey-glass dataloader, example file, metrics to reflect the following task specification:

Dataloader:

• parameterized with L time, should not be calculated each initialization
• Can be locked functionality of dataloader or can be parameters loaded e.g. from csv file:
  • Train 10 L times
  • Test 10 L times (autonomous / autoregressive mode)
  • dt = L time / 75 (dysts)

Metric

• sMAPE, has max value of 200%, so any NaNs -> 200%