speed up the checking of the grad and elec structures (#1649)

* ENH - improve and speed up the checking of the input data structure, especially for CTF headers with hdr.orig
* ENH - add chantype and chanunit to the output grad and elec to ensure that in ft_read_header these are passed to the high-level FT function. This speeds up subsequent checks of ft_dataype_sens
* ENH - speed up the detection of eeg/meg/nirs input by simply chjecking on elecpos/coilpos/optopos. The more elaborate check with ft_senstype is still there, but only used when really needed.
* ENH - remove the elec and grad structure to prevent some unnecessary low-level checks. See also #1551
* ENH - removed extra whitespace between lines of code, nothing functional
* FIX - enabled the persistent-variable check on the input again