[lmi] "-Wundefined-func-template" warnings from clang

[Greg Chicares]

clang-13.0.1-6 gives numerous warnings when this option is enabled.
Here's one example:

In file included from /opt/lmi/src/lmi/custom_io_1.cpp:31:
In file included from /opt/lmi/src/lmi/input.hpp:37:
/opt/lmi/src/lmi/tn_range.hpp:243:5: error: instantiation of function 
'tn_range<int, age_trammel<int>>::read' required here, but no definition is 
available [-Werror,-Wundefined-func-template]
    ~tn_range() override = default;
    ^
/opt/lmi/src/lmi/tn_range.hpp:272:19: note: forward declaration of template 
entity is here
    std::istream& read (std::istream&) override;
                  ^
/opt/lmi/src/lmi/tn_range.hpp:243:5: note: add an explicit instantiation 
declaration to suppress this warning if 'tn_range<int, age_trammel<int>>::read' 
is explicitly instantiated in another translation unit
    ~tn_range() override = default;
    ^
1 error generated.

The intention was indeed to instantiate some things explicitly in
a single translation unit, in the hope of making compilation more
efficient with 1990's compilers on 1990's hardware.

Is this an actual problem that calls for action?