[ENH] Import RandomChannelSelector through init

[TonyBagnall]

imports RandomChannelSelector and adds an example

Also fixed a bug, was selecting based on number of cases not number of channels, not sure how I managed that.

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I have added the following labels to this PR based on the title: [ $\color{#FEF1BE}{\textsf{enhancement}}$ ].
I would have added the following labels to this PR based on the changes made: [ $\color{#41A8F6}{\textsf{transformations}}$ ], however some package labels are already present.

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