Maths convert block proposal

I've issue the pull request for the new maths convert block

https://github.com/mit-cml/appinventor-sources/pull/165

The pull request includes the unit tests

The new block converts:-

• Decimal to Hexdecimal
• Hexdecimal to Decimal
• Decimal to Binary
• Binary to Decimal

Before conversion takes place the input is check to verify it is Decimal Hexdecimal or Binary and if the incorrect input value is enter then the return value is "error"(Note: I was thinking it might be better to return -1 instead of error but I am open to suggestions on this)

Here is a link to my appspot so the new block can be tested  http://mad-robots-ai.appspot.com/

Attached is the companion app needed and a sample project to demonstrate the new block working

Please use the new attached companion app and demo project instead of the one on the previous post as I've made a minor change to the code.

Use these new files with the same appspot as previous http://mad-robots-ai.appspot.com/

good ¡¡¡¡