Add the Variable class with an example

[nilaybhatia]

1. Closes Need a Variable class for intuitively using continuously updated values  #375
2. Working example (will be updated as functionality is refined):

class VariableExample(Scene):
    def construct(self):
        var = 0.5
        on_screen_var = Variable(var, "var", num_decimal_places=3)

        # You can also change the colours for the label and value
        on_screen_var.label.set_color(RED)
        on_screen_var.value.set_color(GREEN)

        self.play(Write(on_screen_var))
        # The above line will just display the variable with
        # its initial value on the screen. If you also wish to
        # update it, you can do so by accessing the `tracker` attribute
        self.wait()
        var_tracker = on_screen_var.tracker
        var = 10.5
        self.play(var_tracker.set_value, var)
        self.wait()

        int_var = 0
        on_screen_int_var = Variable(int_var, "int_var", var_type=Integer).next_to(
            on_screen_var, DOWN
        )
        on_screen_int_var.label.set_color(RED)
        on_screen_int_var.value.set_color(GREEN)

        self.play(Write(on_screen_int_var))
        self.wait()
        var_tracker = on_screen_int_var.tracker
        var = 10.5
        self.play(var_tracker.set_value, var)
        self.wait()

        # If you wish to have a somewhat more complicated label for your
        # variable with subscripts, superscripts, etc. wrap your label
        # string with `$`
        subscript_label_var = 10
        on_screen_subscript_var = Variable(subscript_label_var, "${a}_{i}$").next_to(on_screen_int_var, DOWN)
        self.play(Write(on_screen_subscript_var))
        self.wait()

NOTE: Until #302 is fixed, please use --disable_caching to render the video.

List of Changes

• Add the Variable class to the file ./manim/mobject/numbers.py
• Add the VariableExample class to example_scenes/basic.py

Motivation

Discussed here: #375

Explanation for Changes

Using the Variable class really makes it easy to use continually updated values. See the example above.

Testing Status

I think tests aren't required because the class makes use of updaters and tex, which already have tests.

Further Comments

In the meantime, please feel free to suggest improvements or some more functionalities that can be offered.
TODO:

• Allow a more refined class for the value, for e.g DecimalNumber (with number of decimal places) or an Integer.

Acknowledgement

• I have read the Contributing Guidelines

[leotrs]

Thanks @nilaybhatia , another awesome addition!

[nilaybhatia]

Variable now inherits from VMobject instead of VDict. Please take a fresh look at the updated example (code+gif) in the PR description. @leotrs @Aathish04

[leotrs]

@nilaybhatia I believe this is ready for review, no? I'm unmarking as draft.

[nilaybhatia]

@leotrs Yes, pending the docs and tests. Ready now.

[Aathish04]

One suggestion, and two nitpicks :)

[Aathish04]

@nilaybhatia Awesome! Could you just add a note in the changelog file mentioning the introduction of this object? I totally forgot to mention it in my first review !

stale

[leotrs]

🎉 🎉 🎉