revision of the spec with many changes

for details see changes.rst
slightly changed what is allowed in safe names
brushed up common code, and added better tests
removed cumbersome BOLTS prefix
added name groups to blt spec
rewrote blt.py and added more tests
better error reporting
fixed import failures due to name changes
shortcut notation for identifiers
simpler sanitizing of identifier and substitutions
renamed profile collection to extrusions

to better distinguish from the steel profiles
cosmetics
fixed replacement and better tests
fixed wrong error messages about duplicate standards
equality relation for identifiers and substitutions
fixed imports
converted data to blt format 0.4
added collection id character set check
added a Links class to represent relationships

This will be the building block of a redesigned repository design
rewrote repo