Interface for retrieving datafiles at runtime.
Rename to singular path.

There is going to be only one runtimepath so let's not use paths.
Include data tables instead of Python lookup.

It is better to make this library independent of Python.
Rather than using the python interface and the periodictable
package, the data will now be included in a runtime directory.
Define version major, minor in version.tpl.

version number definition moved from SConstruct to version.hpp file, ie,
its template source version.tpl.  Added macros for DIFFPY_VERSION_MAJOR,
DIFFPY_VERSION_MINOR.
first cut on runtime path resolution function.

Needs testing, may be buggy.
Namespace name is diffpy::runtimepath.
Fix relative path in source directory.
Do unit tests for the runtimepath functions.
Add datadir variable for runtime files.
Extract majorminor version from version.hpp.

Use version.tpl to define major and minor library version.
In each case extract the majorminor tuple of integer versions
and save it in the environemnt.
Load SConscript.version first to get runtimepath.

runtimepath directory contains major and minor version numbers.
Build a list of all data files in the runtime source directory.
Support unquoted DIFFPYRUNTIMERELPATH.

DIFFPYRUNTIMERELPATH macro should be defined in the build script.
When not, fall back to default value comprised from major minor
versions.
Get DIFFPYRUNTIMERELPATH from installation paths.

Define the DIFFPYRUNTIMERELPATH macro according to the data
and lib destination directories.
Add install-data action.

install-data deploys the data files to target runtime directories.
It is executed as part of the default install target.
Store ion list in runtime file.

This is more suitable to have in runtime data file than in constant
string.
Add fstream validator.
Get ion list from runtime datafile.

Electron numbers are now obtained from the ionlist.dat data file.
Store runtime in diffpy/libdiffpyXY directory.
Update of BVS parameters from IUCr.