Mcpl

* Fix time conversion bug in phits2mcpl and mcpl2phits, where ms<->ns
factors had been mistakenly inverted (github issue 53). Thanks to
Douglas Di Julio (ESS) for reporting.
* Reorder link flags in documentation and "fat" Makefile to be correctly
placed after the compilation objects. This fixes modern GCC compilation
using as-needed flag, which is the default on Ubuntu (github issue 51).

* Fix length of all data arrays in Python API in the last particle block
in all files, making pymcpltool --stats work again for files with more
than blocklength particles (github issue 49).
* Python API accepts pathlib.Path objects introduced in Python 3.4 (github
issue 50).

* Introduce support for PHITS. This is done via mcpl2phits and phits2mcpl
command line tools for conversions to and from PHITS binary dump
files. Thanks to Douglas Di Julio (ESS) for help in adding this new feature.
* Introduce new --forcemerge option for the mcpltool and an associated
mcpl_forcemerge_files function in the C API. This makes it possible to
merge particles from otherwise incompatible files, at the heavy price of
discarding descriptive metadata like comments and blobs. This is
intended as a last-resort solution (github issue 43).
* Add new public function mcpl_transfer_last_read_particle, which makes it
possible to perform non-lossy filtering or merging of files using custom
C code (github issue 18).
* Fix pymcpltool --stats sum(weights) bug with python3 (github issue 46)
* Handle unexpected records at the end of SSW headers gracefully (github
issue 45).

* ssw2mcpl: Support MCNP 6.2.

* mcpl.py: Fix issues in MCPLParticleBlock.position() and
MCPLParticleBlock.polarisation() methods which could result in
exceptions or warnings being emitted for certain input files or for
certain versions of python and numpy.

* Correct _POSIX_C_SOURCE to address FreeBSD compilation (github issue 33).
* mcpl.py: Work around issues in numpy v1.14 to fix MCPLParticleBlock.position() and
MCPLParticleBlock.polarisation() methods (github issue 34).
* CMakeLists.txt: Always use absolute rpath (for OSX).
* Fix gcc7 compilation.