[math4] New version of activity: BlockHead

[John Posner]

Hi --

I've just uploaded Version 7 of my "BlockHead" activity to 
activities.sugarlabs.org. As usual, many thanks to Shawn Willden for his 
help and support. This activity is still in the "sandbox", so I'd 
greatly appreciate user reviews!

Here's the activity description:

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BlockHead is a visual calculator, for the addition and subtraction of 
multiple-digit whole numbers. You type in the numbers to be combined, 
and BlockHead draws each one as a set of place-value "blocks" (like 
Cuisenaire rods). For example, the number 528 is represented by a 
5-block in the hundreds column, a 2-block in the tens column, and an 
8-block in the ones column.

You perform the addition or subtraction operation by drag-and-drop-ing 
the blocks:

* For addition, you drag the blocks for both addends to an "answer" 
area, where they are automatically stacked. If the stack height is 10 or 
more, a "carry" button appears; clicking the button performs the carry 
as an animation.

* For subtraction, you drag the blocks for the smaller number on top of 
the blocks for the larger number. BlockHead automatically "chops off" 
the correct amount from the larger block. Sometimes, you need to click 
an "exchange" button, in order to build a stack that is large enough to 
be "chopped off".

At the source code level, you can:

* control the number of columns in each number
* change the number base (any base from 2 to 10)
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Best,
John