Use static_cast to remove conversion warning

[cvvergara]

From:

​https://travis-ci.org/OSGeo/geos/jobs/328675353#L3328

/home/travis/build/OSGeo/geos/src/noding/MCIndexNoder.cpp:154:50: warning: conversion to ‘int’ from ‘size_t {aka long unsigned int}’ may alter its value [-Wconversion]
  si.processIntersections(ss1, start1, ss2, start2);

There are many conversion warnings.

By adding static_cast:

• Keep the original signatures
• Shows a clear intent of the original authors of using a specific type.
• Helps tidy the code
  • C++ is catching type changing see example

Showing intention:

The comments (if any) shows clear intent to the programmer but not to the compiler

Class A {
  /* code here */

  // the user can give me an int and it will be saved on a size_t
  void set_num(int _value) {m_num = _value;}
  // the user will get an int out of the size_t
  int get_num() {return m_num;}
  size_t m_num; 
}

Clear intention is for both the user and the programmer

Class A {
  /* code here */

  void set_num(int _value) {m_num = static_cast<size_t>(_value);}
  int get_num() {return static_cast<size_t>(m_num);}
  size_t m_num; 
}

[cvvergara]

Changes Already done in master