Patch release: maintenance and bug fixes

• New function data_extract() (or its alias extract()) to pull single
  variables from a data frame, possibly naming each value by the row names
  of that data frame.

• reshape_ci() gains a ci_type argument, to reshape data frames where
  CI-columns have prefixes other than "CI".

• standardize() and center() gain arguments center and scale, to define
  references for centrality and deviation that are used when centering or
  standardizing variables.

• center() gains the arguments force and reference, similar to
  standardize().

• The functionality of the append argument in center() and standardize()
  was revised. This made the suffix argument redundant, and thus it was
  removed.

• Fixed issue in standardize().

• Fixed issue in data_findcols().