Refactor color_gradient

[GoThrones]

Overview: What does this pull request change?

This PR refactors the color_gradient method in core.py. It introduces two new parameters: transition_mode and blend_variation for the case when number_of_colors_provided = 1.

Motivation and Explanation: Why and how do your changes improve the library?

The code of color_gradient of main branch doesn't work as intended for the case when number of colors provided is 1. It returns back the same color, same brightness.

The PR makes it possible that when number of color provided is 1, then either the colors can be progressively lighter or darker as chosen, but the color itself will be of same hue. Also the brightness of successive colors can be varied using blend_variation. See the output below:

I used this code to test:

class trycolorgradient(Scene):
    def construct(self):
        #s = Text("India")
        sgroup = VGroup()
        colors = color_gradient([RED], 5)
        for i in range(len(colors)):
            sq = Square().set_color(colors[i])
            sgroup.add(sq)
        self.add(sgroup.arrange(RIGHT))

Here's the output of the main branch: All the squares have same colors and same brightness. That means no gradient at all. Since the method's name is color_gradient, the effect should have gradient.

Here's the output given by PR branch:
When transition_mode = "lighter". Each next square is lighter than the previous one.

When transition_mode = "darker". Each next square is darker than the previous one.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested