Add take_element methods for InMemoryDicomObject

[Enet4]

Allow the user of dicom_object to take the element from the object, removing it in the process.

For in-memory objects, this can translate to less data copying.

    /// <document please>
    pub fn take_element(&mut self, tag: Tag) -> Result<InMemElement<D>> {
        todo!()
    }

    /// <document please>
    pub fn take_element_by_name(&mut self, name: &str) -> Result<InMemElement<D>> {
        let tag = self.lookup_name(name)?;
        self.take_element(tag)
    }

[pjsier]

@Enet4 I'm interested in taking this on! To make sure I'm understanding, would it work to use the remove method of the entries BTreeMap?

[Enet4]

@pjsier Yes, I believe that would be it for the implementation. 👍 A quality PR would also include documentation and a basic test (alongside the ones in the same module).

[pjsier]

@Enet4 sounds great! I can work on that in the next few days