Separate terrain metadata from the data itself.

[JayKickliter]

The PFL format of VecLen, Spacing, Samples... can be a serious performance bottleneck and isn't necessary. It is a bottle neck in that it either mandates that the source of terrain data adds those fields by default, otherwise it requires a reallocation to prepend those two fields. Let's use this current function signature as an example:

int ITM_P2P_TLS(double h_tx__meter, double h_rx__meter, double pfl[], <SNIP>)

we could switch to

int ITM_P2P_TLS(double h_tx__meter, double h_rx__meter, double terrain[], size_t terrain_len, double terrain_step_size, <SNIP>)

Users who have data natively formatted as PFL (side note, what is the origin and meaning of pfl?) can still call the modified function with no overhead:

double * pfl = <externally defined>;
size_t terrain_len = static_cast<size_t>([0]);
double terrain_step_size = pfl[1];
double * terrain = &pfl[2];
ITM_P2P_TLS(h_tx__meter, h_rx__meter, terrain, terrain_len, terrain_step_size, <SNIP>);