New feature - Custom time interval for channel interpolation

[AminAlam]

When interpolating channels of datasets, it is good to have the option of just interpolating a custom time interval of the bad channels instead of the entire time interval. This option is added to the pop_interp.m and eeg_interp.m. The text field for entering the start and stop of the time interval is shown in the interpolation pop page when the EEG dataset is not epoched.