Added reset method to Checkpoint/ModelCheckpoint

[vfdev-5]

Fixes #1422

Description:

• Added reset method to Checkpoint/ModelCheckpoint

trainer = ...

checkpointer = Checkpoint(...)
trainer.add_event_handler(Events.COMPLETED, checkpointer)
trainer.add_event_handler(Events.STARTED, checkpointer.reset)

# fold 0
trainer.run(data0, max_epochs=max_epochs)
print("Last checkpoint:", checkpointer.last_checkpoint)

# fold 1
trainer.run(data1, max_epochs=max_epochs)
print("Last checkpoint:", checkpointer.last_checkpoint)

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[sdesrozis]

It looks good to me 😉