add plot method for eigenvalues

[Daenarys8]

issue: #159

[TuomasBorman]

Commenting wirh phone so little bit hard, but:

Eventhough SCE method, in description of x say that the obkect must be TreeSE (not SCE). Some users might not knoe that TreeSE is actually SCE and this might be confusing

Structure the function so that. One internal functiona prepares the data and another internal function plots the data (.scree_plotter).

Put these show.* parameters as a hidden parameters of .scree_plotter

[Daenarys8]

i have added update to consider actual variance explained(VarExplained) by each PCi in runPCA as it does not return eigenvalues. What other reducedDim are to be supported?

[antagomir]

You could also support MDS (aka PCoA). For other ordinations this is not well defined I think.

[TuomasBorman]

You could also support MDS (aka PCoA). For other ordinations this is not well defined I think.

Also RDA is based on eigenvalues-eigenvectors