further style fixes in point

fix cmake to find TBB installed from source

there was a problem with the FindTBB script for cmake where it cmake was
not looking in paths specified by the environment variable LIBRARY_PATH,
which meant that it was not finding the TBB library files when TBB had
been installed from source and configured with tbbvars.sh

Address gcc 6.1 warnings

add missing cmath header

GCC 6.1 was unable to find cmath in src/point.hpp

Feature/refactor tests directory

Adapt sources to include test_unit.hpp
Add empty CMakeLists.txt to all test type directories

Gtest is currently copy twice!

Fix optional<X&> assignment.

* Correct implementation of operator=().
* Unit test for optional reference assignment.

Modifications to util::optional for KNL compilation

Turn off warning about unused template parameter, when the
parameter is used for SFINAE tests inside the template
parameter list in the json library.

The Intel compiler was unable to compile the
`util::optional` code, and also created some warnings
in the unit tests for optional
* rephrase `enable_if` template argument to use `::value`
  explicitly to work around Intel compiler bug.
* move helper types out of anonymous namespaces in the
  optional unit tests
    * fix warnings about unused member functions
* remove some white space in optional code

rename cl_options.check -> check_and_normalize

Feature/debug trace

* Add `multithreaded()` constexpr function in threading
  namespace.
* Replace `WITH_TBB`-guarded code with multithreadedness-
  checking code.

improved spike chain generation in minapp

- improve robustness of time step and time integration
  interval selection
- changes of the type `if (!id)` to `if (id==0)`.
- fix bug selecting source cell gid when generating
  network connections
- make all-to-all network a user-specified option
    - updated README
- fix bug where an initial spike was not generated for
  the first cell on a communicator when the cell gid
  was a multiple of 20

Defines a TRACE(...) macro that, when enabled at
compile time with -DWITH_TRACE, prints to std::cerr
the location in the source, a time stamp, and the
values of any variables given to the macro.

Uses a mutex to protect against simultaneous writes
to std::cerr when TBB is enabled; it is not
optimized for speed.

Fixed small conflict and fixed compiler warnings about signed-unsigned
comparison when compiling in debug mode.

Conflicts:
	src/fvm_cell.hpp

Feature/swc parser whitespace

Whitespace-only lines are now treated correctly in SWC file parser. This
automatically leads to a consistent and nice handling of old mac-style
eol; they are just another whitespace (not newline).

Refined the test cases a bit.

Added support for mapping swc record types to nest::mc::segmentKind.
This required the adjustment in some tests + the ball_and_stick file, since
the dendrite type of swc record is type '3'. Before, everything except
soma was added (hardcoded) as a dendrite. An swc_parse_error will be
thrown in case of an impossible mapping. Also, no check is performed for
inconsistencies of SWC record types along a branch; the type of the
branch is assumed to be the type of the first record (compartment) in
the branch.

Finally, adapted to the coding style.