Defined the function SlipPlane::calculateDislocationSourceStresses.
Defined generic function that calculates the stress fields on all
defects presenton a given slip plane, due to all other defects present
on the same slip plane.
Defining function SlipPlane::checkDislocationSources
Attention:This function is incomplete.

We require another function which will return all defects that exist on
the slip plane between two given positions p0 and p1, such that they
appear on the list in the order as seen when travelling from p0 to p1.
This function will be defined in the next commit.
Function SlipPlane::findDefectsBetweenPoints defined.
This function looks for defects lying between the two points p0 and p1
provided. The elements of the vector are ordered such that they appear
as they would when travelling from p0 to p1. The function creates a
vector container with pointers to defects lying between two given
position vectors p0 and p1 (both specified in the slip plane co-ordinate
system). The order of pointers within the vector is the one that one
would have when travels from p0 to p1. In the case where there are no
defects lying in between these two points, an empty container is
returned.
Completed definition of SlipPlane::checkDislocationSources
Dislocation dipoles are emitted, the new dislocations are placed so as
to not conflict with existing defects. The new dislocations are inserted
into the slip plane's lists and at the end these lists are sorted.
Suggestion: Maybe a dislocation source should be made inactive if there
are defects lying too close to it.