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.
Things that we must still check for in stage 3:
1) Handling of CONSTANTs:
1.a) "Any program organization unit attempts to modify the value of a variable that has been declared with the CONSTANT qualifier;"
1.b) From table 16.a "The CONSTANT qualifier shall not be used in the declaration of function block instances as described in 2.5.2.1."