Fix

Added diagnostic
Added possibility to post-process regtests

There is a new function that is called after every regtest, before
comparing with reference. Can be used if files produced by plumed
need further post-processing
Autoconf detects zlib
Implementation of zlib

Files with gz extension are transparently compressed and decompressed.

Closes #17
Regtest for zlib

Includes restart with METAD, READ and PRINT, and --multi