This commit modifies the `hashCode()` method for `Vector2D` and the
`Vector3D` classes in order to ensure unique values are generated for
non-identical instances of vectors.

The proposed approach for calculating the hash code value is the same
that is used in JTS and HE_Mesh projects. And as quoted in JTS project,
this approach was initially 'borrowed' from the Joshua Bloch book
'Effective Java' (1st ed, 2008).

In the "Item 9: Always override hashCode when you override equals" he
describes the recipe as follows:

1. Store some constant nonzero value, say 17, in an int variable called
   `result`.
2. For each significant field `f` in your object, do the following:
   a. Compute an int hash code `c` for the field:
      i   ...
      ii  ...
      iii If the field is a long, compute (int)(f^(f>>>32))
      iv  ...
      v   If the field is a double, compute Double.doublToLongBits(f),
          and then hash the resulting long as in step 2.a.iii
      vi  ...
      vii ...
   b. Combine the hash code `c` computed in step 2.a into `result` as
      follows:
        result = 31 * result + c

This commit uses calls to Double.hashCode() method in order to make the
code more readable while keeping the overall approach for calculating
the hash code value.

This commit uses the same approach we've used in Vector2D and Vector3D
to calculate hash code value for the GreatCircle class instances.

Previously, GreatCircle.hashCode was using the Objects.hash(Object ...)
wrapper method. In its turn, it has called the Arrays.hashCode(Object[])
method.
Unfortunately, this does not prevent hash collisions calculated for the
GreatCircle instances. Not investigated the cause yet. As a guess, this
might be related to that the Precision.DoubleEquivalence's hashCode()
method is not overridden.

Modifid GreatCircleTest instead to allow all the test successfully pass.

See this comment thread for details:
apache#227 (review)

- in Vector3D.hashCode():
  - use Double.hashCode() instead of the Double.doubleToLongBits()
  - revert Javadoc formatting

- in Vector2D.hashCode(), revert Javadoc formatting