extract and move pressio-ops to https://github.com/Pressio/pressio-ops

[fnrizzi]

status quo

Pressio/pressio (Pressio upper case is the org name) currently contains the following components:
mpl, utils, traits, expressions, ops, ode, solvers, rom

new solution/approach to evaluate

Goal: splitting all this into two separate repos:

• Pressio/pressio-ops: would contains mpl, traits, expressions, ops
• Pressio/pressio : would contains ode, solvers, rom

Beside a few pros like easier to maintain, separate concerns, one of the main outcomes of this is that all the code in ode, solvers, rom will be generic since it will be written in terms of operations.

constraints to enforce

0. pressio-ops should be a required dependency of pressio: should figure out the best way to fetch it when using pressio. Since pressio remains for now a header-only, we should just fetch via cmake internally to make things easy.

1. we remove utils, and we just move its functionalities where needed: for example, pressio-ops does NOT need the logger so we put it in the pressio repo only

2. for pressio-ops we need to ensure that when installing things parent directories of headers remain the same as they are now: so everything should be installed so that it can included as #include <pressio/roms.hpp> and similar. Obviously, pressio-ops and pressio do not need to be installed in the same place but we should not change the name of the installation toplevel dir for pressio-ops.

3. macros: some PRESSIO_ENABLE_... will inevitably be defined only in pressio-ops or pressio. But we need to make sure these keep the same names and they are used for both.

4. other things that i am missing

how to proceed

we already created https://github.com/Pressio/pressio-ops we extracted the mpl, traits, expression, ops so to move forward we need to:

1. create new branch of the Pressio/pressio repo from develop
2. evaluate how to do the split and use https://github.com/Pressio/pressio-ops
3. go/no-go: on moving forward to make this split official