ENH: Add graph kernel support

ENH: Add TubeGraphKernel graph kernel driver (depends on Boost 1.51)
ENH: Add shortest path kernel
ENH: Add first draft for 'Experiments' directory
ENH: Add VascularAtlasFormation experiment and driver Script
ENH: Added Matt's review comments and WIP on TubeImageQuantiles

ENH: TubeGraphKernel modified with Matt's comments
ENH: AtlasGenerationDriver not dependent on env. variable anymore
ENH: Conditional compilation for Boost related code (TubeTK_USE_Boost)
WIP: TubeImageQuantiles to compute user-specified quantiles
ENH: Added CLI arguments to specify kernel type to VascularAtlasGeneration
ENH: Enhancements of Experiments driver and automatic Midas data download

ENH: Renamed AtlasGenerationDriver.py to ExperimentsDriver.py
ENH: Adapted README file to reflect changes
ENH: Removed 'dir' variables in ExperimentsDriver.py
ENH: Better readability of example config files
ENH: Add Download.py to automatically download data from MIDAS
ENH: Add Pydas-Config.json.example (MIDAS credentials config)
BUG: Bug fix in experiments driver and removal of 'using XXX' directive
BUG: Fixed bug accidentially introduced in TubeDensityProbability
ENH: Added default node labeling support and experiment enhancements

ADD: Support to choose default node labeling (degree, ID)
ADD: Support to choose phantom type in experiments driver
ADD: Graph kernel evaluation, based on full kernel matrix
ADD: Computation of kernel between graph and group-wise summary graphs
ENH: Random seed (e.g., for CVT init.) is passed via ExperimentsDriver.py
ENH: Skip zero-weight edges in WLSubtreeKernel
DOC: Updated documentation in experiments driver scripts
STYLE: Variable renaming for consistency and simple fixes in graph kernels

ENH: Add support for absolute and relative path for loading graphs
ENH: Variable naming for user-given CLI parameters
ENH: Support to omit labeled regions in TransferLabelsToRegions
ENH: Reflect renaming changes in Experiments JSON files
ENH: Added support for global label file when loading graphs
ENH: Support for a global label file
ENH: Refinement of VascularAtlasGeneration

ENH: CLI documentation in TransferLabelsToRegions.
ENH: Adjusted ExperimentsDriver CLI arguments to reflect args of
     the binaries they are passed to.
ADD: Support in ExperimentsDriver to compute global label maps using
     TransferLabelsToRegions.
ENH: Added support to write SVM info files (with #SVs, predictions, etc.)
STYLE: Remove tabs and spaces at EOL
STYLE: Add README markdown files for each folder

Documentation will gradually be added to the markdown files, but for now they
contain a title and a generic footer. Also created a Base/Common folder, into
which some of the files from Utilities will be added, per discussion in meeting.

ENH: Update Cppcheck suppressions
ENH: Suppress CTK manually-specified variables CMake warning
BUG: Fix inconsistent capitalization of -build/-Build directories
ENH: Add README, LICENSE, and .gitignore files