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 see the point in combining read_help and read_hsp if the file formats are really identical, but not for forcing transformation and decimation into the read function.

If you want to make downsampling accessible to users you should make it available as public API (AFAIK users don't call read_dig_points themselves anyways, but various Raw constructors do), so users can `read(); downsample(); write()`. However it's not a great decimation algorithm anyways and that's why it's private and a last resort.