Incorrect formula in OneEuro.h

[edgar-bonet]

A comment at the top of OneEuro.h provides this link to a web page about the 1€ Filter by one of its authors. In that page, line 5 of Algorithm 1 shows that dx is to be computed as per the formula:

dx ← (x − xfilt.hatxprev()) * rate

The code, on the other hand, does (line 60):

dx = (x - xfilt_.xprev) * freq;

According to the provided reference, it should be xfilt_.hatxprev rather than xfilt_.xprev. Which incidentally implies that the xprev member of low_pass_filter is not needed.