Renamed 'date_str' into 'time_str'. Added 'seconds_str'.
Added single parameter overloads to 'derivate1' and 'derivate2'.
Removed implicit constructor from T to vec_t<T>. Added explicit dimension constructor instead.
Added the necessary affectation operators to make it work.
Fixed photinfo for recent code change.
Fixed unit_test for change in vec constructor and derivate1/2 overloads.