Handle enkf run where some realisations are killed

This PR is a first step toward becoming robust for failing realisations. The main parts of this PR are:

1. A datatype 'state_map' which is to record the "state" of a full realisation. The state map is on a per case basis, and is owned an managed by a enkf_fs instance.
2. Code to make record the status of realisations in the state_map.
3. Updates to the core enkf analysis code; it now accepts a mask of which realisations to consider in the update process.

In particular point 2 must be signifiantly improved before we have support for failing realisations in various situations.