MVP: Implement a batch/apply style function

[dougbrn]

Feature request
Blocks #9. We want to apply a function, such as a periodogram function, to all lightcurves stored within a nested structure. We should implement a simple function for applying this function to the lightcurves. With TAPE, this motivated the batch function for grouping the sources and applying a function to them. Our data structures motivate a custom function a bit less, as the data is pre-grouped. So using the base apply may potentially be fine, however we may be able to provide a more intuitive API function for doing this operation and can potentially optimize for how it accesses the lightcurve data under the hood. Usage should look something like this:

mean_flux = nf.batch(my_mean_flux, "my_nested.flux")

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[wilsonbb]

A small suggestion here is that I would avoid the name "batch" here since it has the connotation that we're doing batch processing of splitting the job into multiple computable batches. That connotation makes less sense here where there is no parallelism being implied.