Add visualization options during cn-feature modelling

[genomaxx]

Use feature density barplots and density scatterplots with marginal histograms to show feature distributions. These two options will serve as the pre-modelling visualization options.

Here is an example.

Implement shaded mixture model line plots post-modelling. These plots should be generated automatically when requested for a given signature. Here is an example:

Compose section in new-cn-signatures-vignette on use of these plotting options.

[genomaxx]

PR #47 doesn't include the marginal histogram plotting function. That will need to be a part of other commits and a subsequent PR.