Throw better error for different obs size in minibatcher (eventually resolve)

[odunbar]

Issue

the following assumption that all observations is the same length

EnsembleKalmanProcesses.jl/src/EnsembleKalmanProcess.jl

Line 284 in 959dea5

obs_size = length(get_obs(get_observations(observation_series)[1])) #deduce size just from first observation

Comes out when the user calls update_ensemble!( with different observation sizes. Currently this gives an opaque indexing error. Perhaps we throw an error immediately on creation if obs are not all the same length.

Eventually we should investigate allowing observations of different lengths (and how this interacts with the update_group mechanism.)