TF.has_impedance is slow

[kujaku11]

There is a slowdown or bottleneck in TF.has_{attr}. xarray.sel is currenlty slow, so there may be a better way to test if components are empty or not without using sel. See https://github.com/pydata/xarray/issues/2452. The main issue is that is returning a new data array, which takes time and is not necessary for this function.