Feature/Value.IsZero method

[bpeake-illuscio]

Hello!

Opening this as a draft in case you feel it does not belong in the lib.

When extracting information from Dicom Element values, I often find it necessary to make sure that a value is not "empty" or "zero"; that is that the value provided is an actual value.

A few examples would be a PatientName value of "", a Rows or Columns value of 0, or a OB element with no actual bytes present.

This PR adds an IsZero() method to value that checks if the inner value is actually a zero-value.

Example Usage:

var patientName string
if element.Value.IsZero() {
	patientName = "[UNKNOWN]"
} else {
	patientName = element.Value.GetValue().([]string)[0]
}

This PR will likely have merge conflicts with #195 . If you end up liking both of them, feel free to merge either one, then I can update the other to resolve any merge conflicts that occur.

Thanks!

As a side note: a lot of these PRs are groundwork I am using for a value decoder helper-type I am using internally. If you would be interested in seeing what I am tinkering on there, I could open a draft PR with it, as I think it might make a nice addition to the lib. Let me know!