Fix copying of data product containers

[bocchino]

This PR fixes an issue with copying data product containers. A DpContainer contains an Fw::ExternalSerializeBuffer as a member that provides a view into the Fw::Buffer that it owns. When a DpContainerwas copied via copy construction or copy assignment, the ExternalSerializeBuffer member was not being copied correctly.

I did the following:

• Delete the copy assignment operator from ExternalSerializeBuffer.
• Add two new derived classes of ExternalSerializeBuffer: ExternalSerializeBufferWithDataCopy and ExternalSerializeBufferWithMemberCopy. Each derived class adds just that kind of copy semantics to the base class.
• Revise DpContainer so that the ExternalSerializeBuffer has type ExternalSerializeBufferWithMemberCopy. The slice is owned by the DpContainer, so this is the behavior we want.
• Revise the tests in FppTest/dp to verify that the container assignment is working.
• Make the copy assignment operator of SerializeBufferBase protected, so that assigning into a SerializeBufferBase reference is not allowed.
• Revise a unit test that was assigning into a SerializeBufferBase reference, in a way that was not necessary.

@timcanham This PR pulls out the changes that I submitted to your DpCatalog branch. I thought it would be good to make a separate PR to mainline with just these changes.

[timcanham]

LGTM!

[LeStarch]

One minor comment. Could be fixed on follow-up.

[bocchino]

Note: If we had getSerializeRepr return an object, instead of a reference, then we could return Fw::ExternalSerializeBufferWithMemberCopy as suggested by @LeStarch. I wrote up an issue on it here: #2714.