rebase_agg_windows: Rebase to new linear combination of assets
In benSepanski/pfselect:

Change the price windows and historic price mean windows in trading period i to represent a linear combination of prices, the jth combination being the jth column of the matrix which is the ith entry of new_assets.

rebase_agg_windows(
  agg_windows,
  new_assets,
  asset_rownames,
  scalar_columns_to_rebase = NULL
)

`agg_windows`	A tibble of class `@eval{AGG_WINDOW_CLASS_NAME}` as returned from `aggregate_price_and_mean_windows`
`new_assets`	A list, whose ith entry is a square matrix. Each column represents a new linear combination of the assets to consider replacing one of the old assets.
`asset_rownames`	These are the names of the assets of `agg_windows` in the order which they correspond to the rows of the matrices in `new_assets`
`scalar_columns_to_rebase`	A character vector of column names of `agg_windows` to be rebased. These must be double columns.