Revert "Static variables shouldn't be static."

This reverts commit 643808e06937d36a5a31f3e547b2238fcae8354a.
Add global free functions for dealing with the Chrono data.

While this set of changes do not address the issue of where the data/ directory lives, they offer the following:
(1) it isolates the hard-coded default location (namely "../data"), relative to the location of the executable programs, to a single place (in ChGlobal.cpp)
(2) it allows external projects that depend on Chrono to access the default Chrono data files without having to copy the data/ directory.  Instead, simply invoke SetChronoDataPath() to specify where the Chrono data directory is (for example, in the Chrono source tree or the Chrono install tree)

The three new functions are as follows:
(1) SetChronoDataPath(): specifies the location of the Chrono data/ directory. The default is "../data/".  Note that this function is not thread-safe.
(2) GetChronoDataPath(): retrieves the location of the Chrono data/ directory (thread-safe)
(3) GetChronoDataFile(): obtains the complete filename given the name of a file or subdirectory relative to the location of the Chrono data directory (thread-safe)
Use GetChronoDataFile in all Irrlicht demos.
Merge branch 'develop' into feature/irr_data
Use `const char*` arguments.
Use GetChronoDataFile in demos.