minor enhancements and extensions to non-r-code

[kerschke]

In the C-code of MOP7, x[0] and x[2] were used although in fact only x[0] and x[1] exist. That is x[2] should be called x[1].

In addition, I added an additional functionality for the interface to MPM2. The code now enables to access the covariance matrice of the respective function.

[coveralls]

Coverage remained the same at 89.51% when pulling 68d75fe on kerschke:master into 5562083 on jakobbossek:master.

[jakobbossek]

Thanks. Merging.