Support ! for view access

[omus]

Add support for using ! while using getindex to access a view instead of a copy. For example:

samples[channel, !]

People familiar with DataFrames.jl should already be used to this.

There are probably some good reasons not to do this. If so this issue can be useful for future reference.

[omus]

One reason not to do this is that you can't access a view when accessing a single element (e.g. view(sample, channel, 1)

[jrevels]

this is borderline superseded by #77