histogram: Start plotting after all calculations
histogram: Script functions moved to makefile
histogram: Set 2D histogram ranges by reference file

A reference file is used instead of the minmax file.
histogram: Reference file for 1D histogram fixed

Time unit added for file search in full generality.
scripts: Print all error messages to stderr
histogram: Calculate 1D free energies on full range

The functionality to restrict the range by means of
a minmax file is still supported by calc_fel1d.py.