Modification of the code structure

[JulStraus]

DISCLAIMER: The changes require the merge of PR 9 of TimeStruct.jl.

The reimplementation of EnergyModelsInvestments introduced through Issue 20 led to the desire for a slightly changed design of EnergyModelsGeography. This is achieved through the branch enhanc/emi-reimplement.

All tests in the branch should work individually if you have checked out the branches tf/iterations ofTimeStruct.jl and feature/stor-discharge of EnergyModelsBase.

The following steps have to be conducted before this issue can be closed:

• [x] Update NEWS.md (not included to avoid potential issues with potential rebases)
• [x] Update version number (most likely breaking changes for node development, but not for application)
• [x] Update dependencies
• [x] Improve the documentation

[JulStraus]

Issue closed through (Pull request 21)[https://github.com/EnergyModelsX/EnergyModelsGeography.jl/pull/21}.