def rename_vectors! name_map
      rename_vectors(name_map)
      self
    end
Renames the vectors and returns itself

== Arguments

* name_map - A hash where the keys are the exising vector names and
the values are the new names.  If a vector is renamed
to a vector name that is already in use, the existing
one is overwritten.

== Usage

df = Daru::DataFrame.new({ a: [1,2,3,4], b: [:a,:b,:c,:d], c: [11,22,33,44] })
df.rename_vectors! :a => :alpha, :c => :gamma # df