implement "all or each" approach in values

[rlanzafame]

as many values can be checked using exercise type values, we should allow for flexible (and selective) testing of each value individually. Consider that the following "things" can be defined for each value to be checked:

• type
• tolerance
• tolerance_type
• custom messages, hints, etc

in the current implementation, each of these things are added to the dict as a single value, but could also be specified as a list with same length as values. For example, the following would provide a different absolute tolerance and type for each value in values:

exercise = {
    "type": "values",
    "tolerance": [0.01, 0.1],
    "tolerance_type": ["relative", "absolute"],
    "variables": ['multiplication', 'addition'],
    "values": [13.889, 7.53],
    "types":["float", ["float", "numpy"]]
}

Note the list of lists for the types key, allowing values[1] to have 2 types.