Use `.dataset` rather than `.field_time_serieses`?

I'm wondering if we should replace "field_time_serieses" with "dataset" wherever it appears. I think "dataset" rolls of the tongue better and is also sufficiently distinct from `data` so that it won't be confused.

Also Oceananigans has a prototype "`FieldDataset`" designed to represent a _group_ of `FieldTimeSeries`. It's not used much now, but could be developed further:

https://github.com/CliMA/Oceananigans.jl/blob/main/src/OutputReaders/field_dataset.jl

Search the ParameterEstimocean.jl tree for occurrences of `field_time_serieses` and review how the term is used. Read Oceananigans' `src/OutputReaders/field_dataset.jl` prototype for context before deciding whether `dataset` is appropriate. Done means the naming decision is resolved and any agreed terminology is applied consistently.

Written by the indexing model from the issue text.