Make it possible to predict/simulate any model component

[jbect]

Long term goal: make it possible to predict/simulate any model "component".

For instance, in the case of a standard (noisy) STK model:

• future noisy observations,
• latent process,
• lm component of the latent process,
• zero-mean-GP component of the latent process.

Other examples:

• components of a process defined as a sum,
• real / imaginary parts of a complex-valued GP (see https://github.com/stk-kriging/stk/issues/4),
• partial derivative,
• ...

(ref original ticket on Sourceforge: https://sourceforge.net/p/kriging/tickets/63/)