feat: various improvements for loading files, GH actions, linting, R CMD CHECK fixes

[m-jahn]

• LoadTrack function was refactored to remove code duplications (lapply + bplapply)
• LoadTrack function has now NULL as default chromosome (fixes Non-eukaryotic data and input files? #9)
• handles now binning for bam files regardless if done via bamCoverage or GenomicAlignments
• various small linting fixes
• added author to DESCRIPTION, bumbed Roxygen
• refactored ggcoverage.Rmd and README.Rmd in order to remove all lint errors, adhere to R code style
• added Github Actions workflow to automatically run R CMD CHECK for each commit and PR to dev/main

[m-jahn]

@showteeth can you make a code review? Thanks!

[m-jahn]

@showteeth this PR is ready. fixed many problems and included github actions workflow, with the result that the package now builds without errors on all major platforms, see automatic checks.

I suggest to bump the version of the package to 1.3.0 (minor release). There are no breaking changes as far as I can see.