ENH: installation helpers

----
install-dirs: a general directory installer

  Copy installs non-binary (platform) directories.
  Eg,

      install-dirs -prefix=... -devel
      install-dirs -prefix=... -default -no-app

----
install-platform: a platform (binary) installer

  This is primarily driven by the need to install into system mpi
  directories. The problem noted in issue #1893 is caused by the
  rpm-mpi-hooks (fedora and redhat-8).

  For the additional mpi library qualifier (openmpi-x86_64) to be
  added to the requirements, the mpi-specific libraries (eg,
  libPstream.so) need to be installed in the mpi system directory
  (eg, /usr/lib64/openmpi).

  However, then need symlinks from the system locations back to our
  local directories to ensure that the libraries are correctly found
  via our LD_LIBRARY_PATH and we don't get dummy libraries.

----
update-mpi-links: a post-install update hook

  Expected paths are registered as persistent information into the
  hook during packaging. Triggering the hook after installation
  completes the creation of the symlinks.

Normal usage,
```
prefix="/tmp/local-install/openfoam"

bin/tools/install-dirs -prefix="$prefix" -common
bin/tools/install-platform -prefix="$prefix"
```
Installs
 - /tmp/local-install/openfoam/META-INFO
 - /tmp/local-install/openfoam/bin
 - /tmp/local-install/openfoam/etc
 - /tmp/local-install/openfoam/platforms/linux64GccDPInt32Opt/bin
 - /tmp/local-install/openfoam/platforms/linux64GccDPInt32Opt/lib

Can also place architecture-dependent bits elsewhere,
```
prefix="/tmp/local-install/openfoam"
multi_arch="$(dpkg-architecture -qDEB_TARGET_MULTIARCH)"

bin/tools/install-dirs -prefix="$prefix" -common
bin/tools/install-platform -exec-prefix="$prefix/$multi_arch"
```
Installs
 - /tmp/local-install/openfoam/META-INFO
 - /tmp/local-install/openfoam/bin
 - /tmp/local-install/openfoam/etc
 - /tmp/local-install/openfoam/x86_64-linux-gnu/bin
 - /tmp/local-install/openfoam/x86_64-linux-gnu/lib

Can use it to flatten out platforms entirely,
```
prefix="/tmp/local-install/openfoam"
bin/tools/install-dirs -prefix="$prefix" -common
bin/tools/install-platform -exec-prefix="$prefix"
```
Installs
 - /tmp/local-install/openfoam/META-INFO
 - /tmp/local-install/openfoam/bin
 - /tmp/local-install/openfoam/etc
 - /tmp/local-install/openfoam/lib