add custom digital patterns description
**Choose the label unit from the drop down list or choose your own custom one.
**"Time series / movie" can have Decimal (0.45 sec) or Digital (00:00:00) formats, as well as a custom format the you can type in the given text field.
**"Z-stack" can have length units or a custom format.
**"Spectral" can have units of length or inverse length, or a custom format.