…Intended to be useful for opmon "info" structs

Our normal codegen-ed structs (via ostructs.hpp.j2) have
numeric member variables that are bare instances of the numeric type
they represent. Eg:

```cpp
struct mymodule::Info {
   uint32_t a_counter;
   uint32_t another_counter;
};
```

We pass these structs to the operational monitoring via the
`get_info()` function, which is typically called from a different
thread than the one in which the counters are actually filled. That
means we have to "shadow" each of the members of `mymodule::Info` with
a `std::atomic` which is a member of `MyModule`, and then copy the
value of each atomic into the `Info` object inside `get_info()`. We
can avoid this duplication by holding a `mymodule::Info` as a member
variable of `MyModule` and updating the counters in that object. This
means the counters in the `Info` object have to be atomic, ie:

```cpp
struct mymodule::Info {
   std::atomic<uint32_t> a_counter;
   std::atomic<uint32_t> another_counter;
};
```

We introduce a new pair of moo templates, `OpmonStructs.hpp.j2` and
`OpmonNljs.hpp.j2`, which correspond to moo's existing
`ostructs.hpp.j2` and `onljs2.hpp.j2`. The only real change is making
all member variables that have moo-type "number" into
`std::atomic`. We also have to do a bit of work to make the
`to_json()` and `from_json()` functions play nice with the atomic
variables.

`*.j2` files in the `schema/` directory aren't currently installed by
`daq_install()`, so we install them "manually" in `CMakeLists.txt`

No changes are needed to "info" schema files. But in `CMakeLists.txt`, call `daq_codegen()` for your "info" schema with the "opmon" templates, eg:

```cmake
daq_codegen(mymoduleinfo.jsonnet
            myothermoduleinfo.jsonnet
            DEP_PKGS appfwk TEMPLATES appfwk/OpmonStructs.hpp.j2 appfwk/OpmonNljs.hpp.j2 )
```

In `MyModule`, add a member variable of type `mymodule::Info` (`m_info`, say) and
fill/increment its members in any thread you like. `get_info()` can
then be implemented simply as:

```cpp
void
MyModule::get_info(opmonlib::InfoCollector& ci, int /*level*/)
{
  ci.add(m_info);
}
```