## Summary
This PR updates and improves the Spack package for [UPC++](https://upcxx.lbl.gov).
I'm an LBL employee and developer on the UPC++ team, as well as the maintainer of this Spack package.
### Key Improvements:
* Adding new 2020.3.0 release and support for use of develop/master branches
- Our build infrastructure underwent a major change in this release, switching from a hand-rolled Python2 script to a bash-based autoconf work-alike.
- The new build system is NOT using autotools (nor does it support some of the more esoteric autoconf options), but the user interface for common builds is similar.
* Add explicit support for an MPI optional dependency
- New `mpi` variant enables use of the MPI-based spawner (most relevant on loosely coupled clusters), and the (unofficial) mpi-conduit backend
- This variant is OFF by default, since UPC++ works fine without MPI on many systems, increasing the likelihood first-time Spack users get a working build without needing to correctly setup MPI
* Add support for post-install testing using the test support deployed in the new build infrastructure
* Fix or workaround a few bugs observed during testing
### Status
The new package has been validated with a variety of specs across over seven different systems, including: NERSC cori, ALCF Theta, OLCF Summit, an in-house Linux cluster, and macOS laptops (Mojave and Catalina).
Expose serial/parallel build (MPI), CUDA/OpenMP backends, Clang, and
Ascent bindings.
Interestingly, `warpx +ascent` currently leads to an infinite loop in
the Spack concretizer.
Git-based conditions database for HEP and other experiments.
Use latest release version and current master to support Linux and
macOS. Add core known dependencies and conflicts related to C++17
support.
cxxstd variant added to help transitive dependencies, and for future
support for newer standards in future.
Update WarpX for recent developments:
- add openPMD I/O (default: ON)
- remove electro-static solver option (now a runtime option)
- enable tiny profiler by default
- depend on new CXX std support in make scripts for C++14
- WarpX only supports 2D and 3D in cartesian dims
* davix: add cxxstd variant
Davix is written in C++, so add this variant to allow dependents to specify
this so a consistent ABI is used.
* davix: fix flake8 errors
