Added RETAIN qualifier to VARIABLES.CSV

Variable forcing now uses limited list and buffer instead of systematical instance tree traversal and in-tree "fvalue" to keep track of forced value for pointed variables (external, located). Pointer swapping is performed when forcing externals and located, with backup being restored when forcing is reset. Match Beremiz commit 93ad018fb602.

Fixed __SET_EXTERNAL_FB declaration in accessor.h, invoked when accessing EXTERNAL_BLOCK.VARIABLE. Macro was invoking __SET_VAR with wrong parameter count. Fixed corresponding code generator for ST. Seems similar codegen for IL is also broken, but fix would affect other (unknown) behviours -> didn't touch it.

Remove RETAIN qualifier for F_TRIG and R_TRIG 'M' variable. This is not following standard, and implicitely consumes a lot of RETAIN memory. User can explicitely qualify R_TRIG of F_TRIG instance as RETAIN if needed.

Fix in standard function library: forgot static linkage for inline __str_cmp()

Fixed in standard function library: NE_BOOL was missing.

Fixed in standard function library: String comparison was retrurning wrong result whith strings of different sizes but starting the same, or with zero length strings.

Fixed FIND Standard Character String Function. Now returns 0 in case empty string is given either for IN1 or IN2.

Reworked get_greatest_tick_count computation

Add a column to VARIABLES.CSV, containing variable (derived) type name, additionally to current last column contyaining base type name

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.