NaN filing between partitions with overlap

Created by: zain-sohail

Created a new branch because the one in PR #162 has too many other comments. I think after adding a test, this is good to go.

Changes from the other PR:

• removed progress bar as it has a larger overhead than just calling len
• removed the compute command because len directly gives the metadata access to the num_rows.

To be done:

adding tests with multiple files

• partial data
• one fully nan file (cols)
• consective empty files

last one might fail so we would need null metadata for a more comprehensive treatment https://github.com/OpenCOMPES/sed/pull/162#discussion_r1350242308

also might make sense to keep this as a seperate function or even a generic method that other loaders can use as I think it's pretty useful