Add error message when reading PHASE files.
Make remove_element return index of last element removed.
Implement tracking of child / parent contexts.
Add new function replace_element( ) to util.H
Delete intermediate tokens with only 1 child.

This avoids building up long chains of useless tokens.
Allow changes in non-terminal contexts by switching to a copy first.
Split computation_record out of graph_register.H
Add argument to insert null regs at startup.
Add a garbage collection callback to the pool type.
Merge branch 'virtual-mapping' of github.com:bredelings/BAli-Phy into virtual-mapping