Fix incorrect grouping symbols for Doxygen

Following https://www.doxygen.nl/manual/grouping.html one way to mark groups is by

///@{
...
///@}

Not, however,

//@{
...
//@}

The MR replaces occurences of the latter by the formed and adds a missing @name to the description of the toVector() and fromVector() group. Fixes one sub-issue of #256 (closed).