ERT-745: Using time_map instead of history

1. Introduces the config option TIME_MAP which can be used to load a
   time map manually from a formatted file.

2. The various obs_vector_() initializer functions use a time_map
   instance instead of a history instance to map between dates/days
   and report steps.

The purpose of this commit is to facilitate updates also in situations
where we do not have either an ECLIPSE refcase or a Schedule file.