Use dGEV for return level confidence intervals

[camarois]

1. Fit a dGEV general scaling distribution with Gaussian Copula and Matern Correlation matrix for historical and climex simulated data. Use a constrained delta parameter for MLE optimization based on delta parameter from historical precipitation data from ECCC.
2. Calculate return levels for all durations including sub-hourly, based on scaling properties of the distribution.
3. Calculate confidence intervals with delta method and plot IDF curves.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 6.45161% with 29 lines in your changes missing coverage. Please review.

Project coverage is 80.84%. Comparing base (e2b1e5f) to head (b59174d).

Files	Patch %	Lines
src/plots.jl	0.00%	29 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature/mle-constrained-optimisation #167 +/- ## ======================================================================== - Coverage 83.21% 80.84% -2.38% ======================================================================== Files 17 17 Lines 852 877 +25 ======================================================================== Hits 709 709 - Misses 143 168 +25 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.