* Make `test_common_cells.hpp` and `ball_and_taper.py` agree.
* Add `ball_and_squiggle` model that has a tapering undulating
  profile.

modcc: Support for units syntax within a state block.

Add "lib" to search prefixes for libtbb

Address deprecated use of 'symbol' warning.

Julia 0.5 deprecates use of `symbol` instead of
`Symbol`. This patch just substitutes the
correct call.

Julia 0.5 deprecates use of `symbol` instead of
`Symbol`. This patch just substitutes the
correct call.

* Fix quoting error in library search.
* Add "lib" to prefixes when system is "Linux".

Feature/cable validation

* Implement Rallpack1 validation test (with a workaround
  for inability to set membrane conductance).
* Fix bug in L≠1 case in PassiveCable.jl (this may still be
  wrong).
* Fix bug in peak delta computation in trace analysis when
  both traces have no local maxima.
* Gentle failure on missing `numeric_soma.json`
* Allow multiple `-s` selection operations for `tsplot`,
  acting disjunctively.

* Amend data directory path in validation tests.
* Enmodulate `hh_soma.jl`
* Add HH channel reference data generations script.
* Switch `validate_soma.cpp` to numeric reference data.
* Consolidate common code in `validate_ball_and_stick.cpp`
* Add (nearly) Rallpack1 validation test (see below).
* Gentle failure on absence of reference data in
  `validate_ball_and_stick.cpp`

Can't yet override mechanism default parameter values,
so the cable cell model added to `test_common_cells.hpp`
lets the default stand; validation script will have
to use the default membrane conductance rather than that
given by Rallpack1.

Address bug in fix for cell clone ctor.

* Add test for cell clone semantics.
* Fix `1cell::cell(clone_cell_t, const cell&)` for correct copy semantics.

Fixes Clang 3.7 failure to compile.

Add analytic simple cable solver.

* Use consistent scaling for y[1] scalar voltage in hh_soma.jl
* Also: add more reserved target names to CMakeLists.txt
  helper function.

* Move generation and data to top-level validation directory.
* Make BUILD_VALIDATION_DATA and VALIDATION_DATA_DIR cache vars.
* Add helper CMake functions for data generation.

Note `validation/ref/numeric/foo.sh` is just a placeholder.

* Align defaults with values used in most of the NEURON
  validation scripts.
* Use consistent 100 Ω·m bulk resistivity across both
  NEURON test models and basic validation cells.

Also:
* Ensure intrinsic and passive properties properly set on test cells.

The problem described in previous commit was caused only inside the
cell's copy constructor, where an assignemt for `std::unique_ptr` was
attempted. Since `std::unique_ptr`'s cannot be copy-assigned, I have
made that assignment manually moving the just cloned other's segment to
the cell being constructed. The original `assign_by` method, relying on
STL's `assign()` cannot accommodate this without changing its
semantics (as pointed out by @halfflat), which would then also conflict
with STL's semantics.

Left out a factor of λ in equation 4.

* Expand comments, function documentation in Julia script.
* Use '#' for top level headers in `README.md`

Fix markdown of README.md