Add MWAX_VCS option from_channel and to_channel

[Mouriyan-Rajendran]

Refer: https://github.com/MWATelescope/manta-ray/issues/342

Don't add any new arguments if data range is not selected

parameters to apply channel range: channel_range = true from_channel = integer to_channel = integer

conditions:

• channel_range in a boolean
• from_channel and to_channel are integers with values between 0 and 255 inclusively,
• to_channel must be greater than from_channel