gof (pvalue), fwhm uncertainty, iterative fit, mc energy spec for test fit

[LisaSchlueter]

• gof: pvalue based on beast-squares (baseline), likelihood ratio, MC
• fwhm with correlated uncertainties
• iterative fit in case cov mat not semi positive definite
• function to find nearest semi positive definite cov mat
• MC test data (energy spectrum based on randomized model function)

[codecov[bot]]

Codecov Report

Attention: 43 lines in your changes are missing coverage. Please review.

Comparison is base (2a96714) 0.19% compared to head (041fd42) 18.29%.

Files	Patch %	Lines
src/gof.jl	35.71%	36 Missing :warning:
src/specfit.jl	72.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #32 +/- ## ========================================== + Coverage 0.19% 18.29% +18.10% ========================================== Files 19 21 +2 Lines 1052 1159 +107 ========================================== + Hits 2 212 +210 + Misses 1050 947 -103 ```

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