do.breeding: Breed a pair of individuals
In pwhigham/spatibm: A Spatial Individual-based model for land-based organisms

Breed a pair of individuals, creating a set of offspring. These are placed in space based on displace. Medellian genetics are used to determine the new allele values for the offspring.

1	do.breeding(x, pts, fecund, displace)

`x`	A row from the breeding pairs table, giving the male and female ids.
`pts`	The .ppp population of breeding individuals. This is used to access the (x,y) location and allele values for the parents.
`fecund`	Number of children created if breeding occurs. Defined as two value vector N(m,sd) drawn from a normal distribution. Values always >= 0.
`displace`	Displacement of children drawn from normal distribution around the mid-point in space between the male and female parents.