* It can handle bivariable polynomials of any degree and returns the appropriate attrNames
* Import plySLF (K Vanhoey generic format).
* It can handle bivariable polynomials and spherical harmonics of any degree and returns the appropriate attrNames
* @param filename the file to import;
* @param attrNames reference that will be filled with the attribute names
* the number of attrNames returned depends on the degree of the polynomials :
* the number of attrNames returned depends on the degree of the polynomials / level of the SH :
* - 1 attrName for geometric position (VEC3) : name = "position" ;
* - 3 attrNames for local frame (3xVEC3) : names are "Frame_T" (Tangent), "Frame_B" (Binormal) and "Frame_N" (Normal) ;
* - N attrNames for the function coefficients (NxVEC3) : N RGB coefficients being successively the constants, the linears (v then u), the quadratics, etc. : : a0 + a1*v + a2*u + a3*u*v + a4*v^2 + a5*u^2.
* Their names are : "colorPTM_a<i>" (where <i> is a number from 0 to N-1).
* Their names are : "SLFcoefs_<i>" (where <i> is a number from 0 to N-1).
* N = 1 for constant polynomial,
* N = 3 for linear polynomial,
* N = 6 for quadratic polynomial,
* N = 10 for cubic degree polynomial,
* N = 15 for 4th degree polynomial,
*
* N = l*l for SH of level l,
* ...
* - K remaining attrNames named "remainderNo<k>" where k is an integer from 0 to K-1.