et3 trim/meas_current

[liubenyuan]

Changes:

• A keep_ba (Keep index boolean array) is added to PyEITProtocol dataclass.
• ET3() in pyeit/io/et3.py now use meas_current (default False) option instead of trim.
• A PyEITProtocol object is required for ET3(). The data is trimmed using protocol_obj.keep_ba when meas_current is False.
• Minor tweak/correct the examples in example files.