differentiator: util-tree: add "g=9.81" to the list of constants.
util: add read_into_vector_errors(), variant of read_into_vector() for "value - error" datasets.

Maximal error among dataset values is taken as effective.
stddev: get variable name for machine output from OUTPUT= environmental variable.
stddev: support datasets with errors, enabled by INPUT_ERRORS= environmental variable.
differentiator: main-error_calculator: support terse and machine output, enabled by TERSE= and OUTPUT= environmental variables.