    util.parseCssNumeric(value)

Parse the provided value as a float and return an object with the number as the `value` parameter. If the value cannot be converted into a number, return `null`.

If the number is followed by a unit, assign it as the `unit` parameter in the return object. If there is no unit, assign an empty string. Examples:

    joint.util.parseCssNumeric('hello'); // => null

    joint.util.parseCssNumeric(1.1); // => { value: 1.1, unit: '' }
    joint.util.parseCssNumeric('1.1'); // => { value: 1.1, unit: '' }
    joint.util.parseCssNumeric('1.1px'); // => { value: 1.1, unit: 'px' }
    joint.util.parseCssNumeric('1.1em'); // => { value: 1.1, unit: 'em' }

    util.parseCssNumeric(value, restrictUnits)

Parse the provided value and only accept it if its unit is part of the `restrictUnits` parameter; otherwise return `null`.

The `restrictUnits` parameter can be a string or an array of strings. If you provide an empty string, that means you are expecting `value` to have no unit. Providing an empty array always returns `null`. Examples:

    joint.util.parseCssNumeric(1.1, ''); // => { value: 1.1, unit: '' })
    joint.util.parseCssNumeric('1.1', ''); // => { value: 1.1, unit: '' })
    joint.util.parseCssNumeric('1.1px', ''); // => null
    joint.util.parseCssNumeric('1.1px', 'px'); // => { value: 1.1, unit: 'px' });
    joint.util.parseCssNumeric('1.1px', 'em'); // => null

    joint.util.parseCssNumeric(1.1, []); // => null
    joint.util.parseCssNumeric('1.1', []); // => null
    joint.util.parseCssNumeric('1.1px', []); // => null

    joint.util.parseCssNumeric(1.1, ['']); // => { value: 1.1, unit: '' })
    joint.util.parseCssNumeric('1.1', ['']); // => { value: 1.1, unit: '' })
    joint.util.parseCssNumeric('1.1px', ['px']); // => { value: 1.1, unit: 'px' });
    joint.util.parseCssNumeric('1.1px', ['em']); // => null

    joint.util.parseCssNumeric(1.1, ['', 'px']); // => { value: 1.1, unit: '' })
    joint.util.parseCssNumeric('1.1', ['', 'px']); // => { value: 1.1, unit: '' })
    joint.util.parseCssNumeric('1.1px', ['', 'px']); // => { value: 1.1, unit: 'px' })
    joint.util.parseCssNumeric('1.1em', ['', 'px']); // => null