add default NULL value to references (as suggested by anonymous in issue 62)
(Not really needed, but it is good practise...)

merge

fix issue #73: add missing overloaded functions ADD, SUB, MUL and DIV for TIME, TOD, DT, ...

remove debugging printf() introduced in previous commits

merge

fix bug introduced in previous commit (force literals to unsigned log long)

return storage of task interval back to 1 ns resolution (when calculating least common multiple et. al. of task intervals).

addendum to issue #76: detect overflow when calculating least common multiple of task intervals.

definitive fix of issue #76: remove internal overflow of u64 when calculating least common multiple of task intervals.

Test for overflow when translating task periods/intervals to integer variable.

partial fix to issue #76 - calculate task periods with 1ms (instead of 1ns) resolution. Allows for tasks with periods larger than 4.295s, but smaller than 50 days.

partial fix for issue 74: allow deref of REF in left hand side of assignment (only works in FUNCTIONS for now)

fix issue #78: IL NOT operator not working

fix issue#77 - when IL FB followed by SFC FB, non-existant error was detected in IL code.