fix(get_at_distance returns the first index): get_at_distance returns the first index no matter the input distance

[gambon2010]

The way the assignment and comparison are combined leads to an incorrect evaluation.

The line is trying to both assign and compare in a single statement, which results in incorrect logic. The assignment should be done separately from the comparison.

[o-murphy]

@gambon2010 Thanks, dude It also will work in oneline if add the brackets like

        if (i := self.index_at_distance(d)) < 0:
            raise ArithmeticError(
                f"Calculated trajectory doesn't reach requested distance {d}"
            )