ARM Fixes

See merge request !734

Few changes for ARM ThunderX2 and ARM-clang:


Do not assume char is signed. Use int8_t for the time-bin .
Improve the detection of the ARMv8.1a cycle counter assembly instruction.
Fix cycle.h for ARMv8.1a. Will push upstream to FFTW.
Add a synchronization point in the memswap() for clang on ARM.
Do not let the autotools choose the flags for clang and GCC on ARM 8.1a. Use the native instructions.
Upgrade to some of the autotools macros.

Implement an optional minimal smoothing length based on the softening of the particle

See merge request !732

Update the IPO support for GCC and CLANG.

Closes #539

See merge request !735

Note that you may need to define "AR", "LD" and "RANLIB", if the names do not
match the default ones (clang/llvm are often trailed by version numbers).

Star formation following Schaye08

See merge request !705

Add some comments to the random number generator to make sure we don't change the numbers by mistake.

Planetary documentation

See merge request !733

Removed duplicate line in the script dowloading planetary EoS files. End of lines in the Uranus REAMDEs.

This should have been done as it includes the system flags for selecting POSIX, for instance

Removed the dependency upon pysphviewer in the solution script for the star-forming isolated galaxy.