Manual mask names

[camisowers]

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What is the purpose of this PR?

Closes #1125.

1. Prevents duplicate names from causing issues while generating the cell table.
2. Theres currently also a bug in the nuclear expression logic.
   Currently if nuclear_counts=True, the nuclear counts are calculated in a loop for each mask (large/unnecessary computation burden for many ezseg masks).
   The cell table creates it's own set of rows for the nuclear masks according to the ezseg logic, while also having a second set of columns suffixed with _nuclear using traditional cell table logic. We want to remove the duplicated nuclear expression values, as well as saving the computational effort.

How did you implement your changes

1. Adds mask_types as an optional argument to generate_cell_table() detailing which masks to extract expression data for; the default is set to ['whole_cell'].
2. Ensure nuclear counts are only done once, when the whole cell masks are being processed.

Remaining issues

Add manual mask name specification into the ez segmenter notebook. @bryjcannon

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[srivarra]

Looks good!!

[bryjcannon]

Code looks good with, just need to update the example_dataset and can then merge with main.

[srivarra]

@camisowers Dataset is updated, this is good to go.