Observation noise sum to evaluated function

[alsgregory]

• Change the way likelihood is calculated by making a function with the constant measurement error variance on all data points and then use that in likelihood.

• [ ] Make a DG0 function with all cell centres set to R and then project this to the ensemble space and use it in weight calculation. more of a weight update procedure than an observations change.

• [ ] Change demos to match this way of finding observations.

• Need to make it so that the observations can be computed by user by just summing together evaluated function at coordinate and measurement error, not to basis coefficient.

[alsgregory]

closing because if we have independent observation error over each observations, observations coming from the same cell will have non-correlating error. must remember to say that this is how we calculate observations in documentation.