rename app/regrid-xy AND have it use calls to fre-cli's regrid instead

[ilaflott]

app/regrid-xy has been refactored into a fre-cli app, where it's strucutre as a smaller, more modular function renders it far more testable in github's CI/CD. The calls to that tool look like fre app regrid <OPTIONS/INPUTS ETC>.

Successfully closing this issue entails replacing the current fre-workflows/app/regrid-xy functionality with fre app regrid, AND removing old code that no longer belongs in this repository