Add an int constructor for OPENFLUID_InitializeVariable

[Arthoni]

Currently handles double and long but since C++ default integer type is int a syntax like
OPENFLUID_InitializeVariable(uPtr, "var.a", 0) will result in a compilation error caused by ambiguous cast.

[Arthoni]

Proposal branch: https://github.com/Arthoni/openfluid/pull/new/openfluid_macros_int
To be discussed with @crevoisier if any side effect could happen (eg related to some implicit cast cases)