Math activity

A math activity computes an arithmetic expression using the message's attributes. The expression must return a number. For example, given the following input message:

{
    "tempF": 50,
}

after processing by the following math activity:

{
    "math": {
        "name": "MyMathActivity",
        "math": "(tempF - 32) / 2",
        "attribute": "tempC",
        "next": "MyDatastoreActivity"
    }
}

the resulting message looks like:

{
    "tempF" : 50,
    "tempC": 9
}