Support decimalPlaces in the computed validation routine.

[chrisala]

Currently an expression based validator of the form:
{
"param": {
"expression": "totalSurveyBaseline",
"type": "computed"
},
"rule": "max"
}
is supported.

However, for a recent use case, we need the validation to compute the value to 3 decimal places as rounding down will cause an incorrect triggering of the max. e.g. the value evaluates to 0.083 which is rounded down to two decimal places at 0.08 meaning a (correct) set of values between 0.081-0.084 will be flagged as invalid due to rounding.

We should support a "decimalPlaces" field as used elsewhere:
{
"param": {
"expression": "totalSurveyBaseline",
"type": "computed",
"decimalPlaces":3
},
"rule": "max"
}