ENH: make read_raw_segment into a method called read_segment

This function is currently only being used with the raw class. It seems that it is better used as a method. This will be especially useful for subclassing the Raw class (e.g. kit). WIP is to implement preload as an option for a gui to reduce headshape points methodically, select marker points in the event of one having fallen off (which happens more often than you would expect), check dev_head_t, etc.
Read_segment would be necessary for using python32 since the arrays are too large otherwise. 32-bit is needed for the WIP GUI or for computers running that version. It will also allow the conversion utility to be more resource conscious.