Fix R dependencies for gsl and reticulate in the METviewer Docker image.

[JohnHalleyGotway]

Describe the Problem

This issue arose via MET-Help:
https://rt.rap.ucar.edu/rt/Ticket/Display.html?id=97638

The user reported that when running the METviewer container, the plot fails whenever aggregation methods are requested. I replicated this behavior and produced the following R-script error message:

RunningRscript /opt/tomcat/webapps/metviewer//R_work/include/agg_stat.R
==== Start Rscript error ====
Error in library(gsl) : there is no package called 'gsl'
Execution halted
==== End Rscript error ====
Rscript time 0:00:00.212

So debugging reveals that both the gsl and reticulate libraries are missing from the installation of R inside the container. Both are included by at least one of the METviewer R-scripts.

The issue with GSL is that yum install gsl-devel installs GSL version 1.15. However, the R GSL package requires GSL version 2.1 or higher. So fix the system level installation of GSL and add one line to also install the missing reticulate package.

Expected Behavior

The aggregation methods should work inside the container.

Environment

Describe your runtime environment:
1. METviewer Docker image

To Reproduce

Describe the steps to reproduce the behavior:
*1. Follow the METplus Training video: https://dtcenter.github.io/METplus-Training/modules/METviewer/docker.html
2. Launch METviewer and load sample data.
3. Make any plot that uses aggregation.
4. Note the error in the Rscript log tab.
Post relevant sample data following these instructions:
https://dtcenter.org/community-code/model-evaluation-tools-met/met-help-desk#ftp

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