Resinsight improvements jira94

This pull request mainly concerns improvements for Resinsight:

1. Fixed a bug in libecl_well for amalgamated LGR wells.
2. Added a new ecl_grid_dims structure for fast queries/lookup of grid dimensions.
3. Added a new flags option to ecl_file_open(); mainly intended to signal that the stream should be closed between each time it is accessed (to reduce the number of open file descriptors).