NF: add constants for presence of data packages
NF: add constants for presence of data packages

HAVE_EXAMPLE_DATA and HAVE_TEMPLATES constants indicating presence of
eample data and templates packages.

alexis-roche/niseg,arokem/nipy,alexis-roche/register,nipy/nipy-labs,alexis-roche/nipy,arokem/nipy,bthirion/nipy,alexis-roche/register,arokem/nipy,alexis-roche/nipy,bthirion/nipy,nipy/nireg,alexis-roche/register,bthirion/nipy,alexis-roche/nireg,nipy/nireg,alexis-roche/nipy,arokem/nipy,alexis-roche/nipy,nipy/nipy-labs,alexis-roche/niseg,alexis-roche/nireg,bthirion/nipy