Add helper function for checking if a channel is open

[crodriguezvega]

Similary as for #3466, create an IsOpen function in the Channel type that does the open check:

if !channel.IsOpen() {
  return errorsmod.Wrapf(channeltypes.ErrInvalidChannelState, "expected %s, got %s", channeltypes.Open, channel.State)
}

func (channel Channel) IsOpen() bool {
  return channel.State == OPEN
}

And use it everywhere where we check if the channel is open or not.

[DimitrisJim]

reminder comment: needs to be changed in 04-channel branch too.