Print warning when overwriting COPY EXTEND %{%} block from instance

[willend]

Candidate solution to #1785

The attached instrument COPY_EXTEND_1 has a COMPONENT comp2=COPY(comp1) instance with an EXTEND %{%} block, which thus overwrites the EXTEND %{%} that arrives via the COPY(comp1) clause:

mcstas COPY_EXTEND_1.instr 

-----------------------------------------------------------
WARNING (COMPONENT comp2): Existing (COPY) EXTEND block:

  EXTEND %{
    // An extend block
    dummy=_comp->index;
  %}

will be been overwritten by:

  EXTEND %{
    // An other extend block
    dummy=2*_comp->index;
  %}
-----------------------------------------------------------

-----------------------------------------------------------

Generating single GPU kernel or single CPU section layout: 

-----------------------------------------------------------

Generating GPU/CPU -DFUNNEL layout:

-----------------------------------------------------------
CFLAGS=

I further attach COPY_EXTEND_2 which has a COMPONENT comp2=COPY(comp1) with an EXTEND %{%} block, but where COPY(comp1) includes no EXTEND block:

mcstas COPY_EXTEND_2.instr 

-----------------------------------------------------------

Generating single GPU kernel or single CPU section layout: 

-----------------------------------------------------------

Generating GPU/CPU -DFUNNEL layout:

-----------------------------------------------------------
CFLAGS=

[g5t]

This change accomplishes the goal to warn users when they overwrite an inherited component instance EXTEND block set via COPY.
I think the message could be clearer, but my opinion is not necessarily correct.