Introduce in-place operations for data processing

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For instance: stressfunction!(d,step) smooth!(d,2)

Although RheoDataTime/Freq objects are immutable, arrays can be manipulated with push!, pop! and append! and already allocated values changed.

There would be marginal differences in computational costs since array data is already managed efficiently (i.e. not deep copied). It is just about the syntax and making it consistent with a typical workflow.

[akabla]

This has been done for data generation. Pushed to next version for in-place processing.