make_meg_bids.py should automatically check for .txt file existence in brainsight folder

[witherscp]

I noticed that make_meg_bids.py does not automatically check the Brainsight folder for the Exported_Electrodes.txt file in the absence of the -mri_bsight_elec argument. I think an easy fix will be to add in an argument for searchpath in the call to write_mne_fiducials() in lines 201-205 of make_meg_bids.py.