add an index that marks the start of the valid range of the virtual array
ref: rename method
only for a level 0 block my the end pointer be set to a value different from the blocks capacity
ref: make methods private
method to retrieve nr items put into lvl 0 block
remove default argument (which was not the default anymore...)
ref: rename method
check_blocks now allows to specify block where checking stops
reorder sequence of dead blocks if neccessary
minor simplification
handle dead blocks at beginning of linked list
store block nr in block instead of maintaining extra variable
TODOs
obtain iterators more efficiently
reduce virtual array capacity from start of linked list