Explore the possibility of immutable input data

Currently, prospect modifies its inputs, including setting masked flux values to NaN. These changes are visible to users, especially in a notebook setting. It would be cleaner to have prospect make copies of the data it needs to modify. This could be done, for example, when transferring the input data to column data stores that Bokeh takes as input.

No files or tests are named. Start by tracing where input data is transferred to the column data stores used by Bokeh and where masked flux values are set to NaN. Done means prospect can modify visualization data without changing the user-provided inputs, including in a notebook.

Written by the indexing model from the issue text.