MRG, ENH: Speed up NIRx read without preload

[larsoner]

Closes #7744

@rob-luke this code only takes 1 sec now, no idea how long it takes on master because I lost patience waiting after 30 seconds or so:

import time
import mne
fnirs_data_folder = mne.datasets.fnirs_motor.data_path()
fnirs_raw_dir = fnirs_data_folder + '/Participant-1'
raw = mne.io.read_raw_nirx(fnirs_raw_dir)
t0 = time.time()
raw.save('/dev/null', verbose='error')
print(time.time() - t0)

[kylemath]

This change is helpful for us and we will add to our import function too (our data files for different blocks sometimes have different sized headers)

[rob-luke]

Thanks @larsoner, I appreciate you fixing this.

[larsoner]

Okay, in it goes, thanks for looking @kylemath @rob-luke . In the future feel free to click the green "Review changes" in the upper right of the files-changed page and "Approve" or add some comments and "Request changes" etc., too, if you want. It's always good to get extra eyes (and know that there have been extra eyes) on any code changes...

[rob-luke]

feel free to click the green "Review changes" in the upper right

Thanks, will do in the future.