Add ninjaCap whole head probe as a loadable default template

[harmening]

Data are stored in cedalion/data/ninja_cap_probe. This includes

• fullhead_56x144_System2_optodes.tsv: optodes positions and labels,
• fullhead_56x144_System2_landmarks.tsv: EEG landmark positions and labels,
• fullhead_56x144_System2_measlist.pkl: small measurement list (including only sourceIndex, detectorIndex, wavelengthIndex, source, detector, channel, wavelength).

Functions:

• cedalion.io.load_tsv: loads from tsv file and returns xr.DataArray.
• cedalion.datasets.get_ninja_cap_probe(): directly returns optodes, landmarks and measurement list.

Also added: test_load_tsv() and some missing doc strings in the files I touched.

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To load the whole head ninjaCap simply do

import cedalion.datasets
geo3d, landmarks, meas_list = cedalion.datasets.get_ninja_cap_probe() 

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@lauracarlton, @avolu, please let me know if we need further information of the original fullhead_56x144_System2.SD or if we are good with this for now.