rename impute_below to impute_below_all

[njtierney]

impute_below would work on a vector - which makes it an alias to shadow_shift, which is kind of annoying, but is just due to the evolution of the package.

[njtierney]

This is not mission critical at the moment, will move to Version 0.4.0

[njtierney]

Current progress on this is on the branch impute-below-all

Looking at this now, it will make sense to soft-deprecate shadow_shift in the next version 0.5.0, and then remove it completely later in the piece. I think that it is more useful to think of impute_below as an imputation, rather than shadow_shift as a special operation.

[njtierney]

Need to re-write the tests for impute_below as it used to operate on a dataframe