Feature/fvm multicell

* move from index_view to const_index_view for mechaninism
  inintialization. This allows us to pass more generic container
  types to the constructor while avoiding redundant copies
* NOTE: required update of
    * external/vector
    * external/modparser
* fixed signed-vs-unsigned comparison warning in algorithims.hpp

Note: use of non-const reference in `mechanism_catalogue::make`
is an interim solution.

small edits to pid document

add planning documents

* Fix bug in 'close' event delivery for multicell.
* Fix displacement scaling bug in gather_all_with_partition.
* Command line parameter io, reporting for new miniapp options.
* Only master rank performs command line parameter writing.
* Add test for mpi::gather_all and mpi::gather_all_with_partition.
* Extend utility of `util::assign`, `util::append`.

Fixes handling of empty ranges and ranges with l>r

merge master into my fork

Adds support to call `std:distance()` in cases where a valid conversion
between iterator and sentinel exists.

* Include new option -g in miniapp to specify cell group size.
* Miniapp -r option generates a ring network.
* Modify EXPECTS() so that when assertions are disabled we
  avoid unused variable warnings.

Compatibiliy fixes for gcc 4.9.2, clang 3.5

Bugfix: partition in serial gathered vector

* Consolidate basic cell models used in unit and validation tests.
* Test `fvm_multicell` with same tests as `fvm_cell`, plus
  some index tests for the multiple cell case.

See defect report:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1467

Note: failed assertion at run time with both fvm_cell
and fvm_multicell.

This new `util:distance()` operating on an iterator `I` and a sentinel
`S`. It reverts to `std::distance()` if `I==S`, otherwise it increments
iterator sequentially until it reaches the sentinel and returns the number of
total increments.

Also removed tests for ranges with `l>r`.

Also added more unit tests for testing ranges of counters.

Still open: handling of l>r cases.

Also adresses the problem of using `std::distance()` with different
iterator and sentinel types.

Bugfix/gcc4.9.3 compatibility

Latest Multicore Optimizations

Minor fixes for PR#86 branch.

* Count tests with int not bool in `mpi_listener.hpp`
* Ytpo fix in `gathered_vector.hpp`
* Add missing include in `test_algorithms.cpp`