Rename all the badly named functions in each module and add unit tests

[axiezai]

List of things that need to be renamed:

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• filters.py/get_freq_spectrum_notdict - incorporate this as boolean option in get_freq_spectrum, and rename to get_frequency_spectrum
• permute.py/reorder_connectome - this function seems oddly specific to desikan-killiany ordering, maybe rename to something related to DK.
• there's a replicate get_freq_spectrum in preprocess.py
• maybe it's worthwhile to rename preprocess/preprocess.py
• we should consider removing read/data_reader.py all together, they are too specific for our use case
• eigenmode.py needs an overhaul, @axiezai
• laplacian.py needs renames for functions that gets complex/regular and one specific function for spectral graph model that needs frequency as an input
• network_transfer.py has 2 network transfer functions, need to do the same as laplacian for the alpha parameters
• run pylint after everything is done

[axiezai]

As discussed with @pfdamasceno , we should separate laplacian computation from network transfer functions, the network transfer functions should take the laplacians and eigen vectors as an input that's also a part of the Brain object.

[axiezai]

Updated laplacian.py in e4df204, now there is one function for both parameter settings, however, still need to combine real + complex laplacian function

edit: real + complex laplacian functions will stay separate