Tb polhemus

This is WIP.

Adds function apply_dig_points to apply hpi, cardinal, and dig points to the Info.

Remove unnecessary import functions for np.loadtxt.

Needs test. Not ready for review
I used `apply_dig_points` because I wanted to removed the occurrences of the pattern matching for numpy.loadtxt function. I actually just updated the function so that it can take the elp and hsp txt files directly so that there is no separate steps. since this code is shared between kit and brainvision and potential any system that using polhemus, I wanted to reduce the duplications across the package.