Extend localisation with methods suitable for GEN_OBS type of observation nodes

[oddvarlia]

Use case and application:
Be able to setup localisation with correlation groups including individual observations from an GEN_OBS node and not include all observations for a GEN_OBS node.

Use of timelapse geophysical observations having spatial (and temporal) location and correlated to FIELD parameters and where the number of observations is moderate (< 50 to 100). The method is not feasible with large number of observations since one correlation group is created for each observation. At least one need to check the performance with increased number of observations in the GEN_OBS node.

Description of extensions of the localisation code:

1. Be able to specify individual observations from an GEN_OBS node by node_name:index
2. Be able to (as before) specify a GEN_OBS only by its node name.
3. Let the user choose between 1. or 2. above and depending on the choice define the valid list of observation names (used by the ERT instance) either as a list of node names or a list of node names for non GEN_OBS observation nodes and in expanded form as nodename:index for GEN_OBS type nodes.
4. Extend the specification of one correlation group to make it possible to automatically expand the specification to N correlation groups, one per observation in the GEN_OBS type observation nodes.

Example showing two new keywords, create_multiple_correlation_groups and obs_coordinate_file.
When create_multiple_correlation_group is true, this correlation item is expanded to N correlation items, one per observation in the gen_obs_A node which here MUST be of type GEN_OBS. The use of the create_multiple_correlation_groups is a special case since it implies not only that this correlation group is expanded into N correlation groups, but also that it set some additional requirements for the input. The specified obs_group should in this case be a list of only one observation node name of type GEN_OBS. Furthermore, the specified observation coordinate file must be a column based file with X and Y coordinates and a one-line heading. The number of coordinates must match the number of observations in the GEN_OBS type node specified.
correlations:
-name: some_name
create_multiple_correlation_groups: True
obs_coordinate_file: "gen_obs_coordinates.txt"
obs_group: ["gen_obs_A"]
param_group: ["field_F"]
field_scale:
method: exponential_decay
main_range: 1300
perp_range: 500
azimuth: 10

It is also related to equinor/ert issue number: 2215

[oddvarlia]

After some more check of the existing API functionality, I think we have what is needed to to this job.

[berland]

Given that #363 was closed, should we keep this open? @oddvarlia