Don't print correlations when they are NaN

When correlation between parameters are NaN `fit_report` prints them. For example the output of [this script](https://gist.github.com/tritemio/35854ac8f0a9a6be234b) is: 

```
[[Variables]]
     ampl:         6884.602 +/- nan (nan%) initial =  7000
     baseline:     22.57765 +/- 1.745224 (7.73%) initial =  24
     offset:       2.753906 +/- nan (nan%) initial =  2.8
     tau:          2.766161 +/- 0.02408554 (0.87%) initial =  2.8
[[Correlations]] (unreported correlations are <  0.100)
    C(ampl, tau)                 =  nan 
    C(ampl, baseline)            =  nan 
    C(baseline, tau)             = -0.419 
    C(ampl, offset)              =  nan 
    C(baseline, offset)          =  nan 
    C(offset, tau)               =  nan 
```

Make sense to print only the meaningful correlations. With the present PR, `fit_report` will skip all the correlations that are NaN. In the previous example the output becomes:

```
[[Variables]]
     ampl:         6884.602 +/- nan (nan%) initial =  7000
     baseline:     22.57765 +/- 1.745224 (7.73%) initial =  24
     offset:       2.753906 +/- nan (nan%) initial =  2.8
     tau:          2.766161 +/- 0.02408554 (0.87%) initial =  2.8
[[Correlations]] (unreported correlations are <  0.100)
    C(baseline, tau)             = -0.419 
```