Add shell tool

[hiker]

We can't use say $(nf-config -flibs) as option for a linker (since we are not invoking subprocess with a separate shell).

So a linker might need to run the above command to get the right set of options required. ATM in various build scripts in lfric_atm-fab we have a shell tool to do that, it would be good if this could just become part of the ToolBox to remove duplicated code.