Linearize a trigonometric expression : tlin

tlin takes as argument an expression containing trigonometric functions.
tlin linearize products and integer powers of the trigonometric functions (e.g. in terms of sin(n.x) and cos(n.x))
Examples

• Linearize cos(x)*cos(y).
  Input :
  tlin(cos(x)*cos(y))
  Output :
  1/2*cos(x-y)+1/2*cos(x+y)
• Linearize cos(x)3.
  Input :
  tlin(cos(x)^3)
  Output :
  3/4*cos(x)+1/4*cos(3*x)
• Linearize 4 cos(x)2 - 2.
  Input :
  tlin(4*cos(x)^2-2)
  Output :
  2*cos(2*x)