* Set OS-specific default compilers

* Fix flake8 warnings

- Added new preferences to config.yaml:
  - source_cache
  - misc_cache
  - verify_ssl
  - checksum
  - dirty

- Moved temp finding logic to spack.stage
- Updated stage tests
- Added tests for new path substaitution of $user, $spack, $tempdir

- Added a schema for config.yaml
- Moved install tree configuration to config.yaml
- Moved etc/spack/install.yaml to etc/spack/defaults/config.yaml
- renamed install_area to "store", to use a term in common with guix/nix.
  - in `config.yaml` file, it's called the `install_tree` to be more
    intuitive to users.
  - `install_tree` might've worked in the code, but `install_tree` is
    already a global function in the spack namespace, from
    llnl.util.filesystem.

* py-pil: Does not build with Python3.

* Set py-pillow to be the default pil provider

* Update package.py

* Change to comments requested by adamjstewart

* Remove version constraint from extends(), avoid a Spack bug.

Not compiling due to mpi error. Also getting this error from the command line so could be separate issue. Otherwise package definition first draft complete.

Added variants and dependencies to plumed but I'm getting an error when trying to resolve mpi to a valid virtual package

- Default scope is versioned with spack and can be overridden by site
  or user config.

- Default scope provides sensible default concretization preferences
  for all of Spack.

- per-platform concretization scope can be added later (to force a
  particular MPI on, e.g., Cray systems)

The share/man path should be in prefix_inspections for MANPATH as
share/man is a common location for manual pages.

Fixes #1072.