fix issue 782: improvement to gp-fit.stan

#### Summary:

Fix issue #782 

#### Intended Effect:

The model in section "Fitting a Gaussian Process" of the manual is written in a more efficient way, so let's use it. It's also a good way to check that the model written in the manual is  indeed correct.

#### How to Verify:

Should give the similar to old version

#### Side Effects:

None.

#### Documentation:

None.

#### Reviewer Suggestions:

Anyone