Issue Tracker for GemPy 2.3

The most critical issues are:

• Incompatibility with pandas>1.3.4 (pandas version should thus be set >1.3.4 in the requirements)
• Incompatibility with numpy>1.2x due to lack of theano maintenance --> replacing theano
• Incompatibility with more recent pyvista versions as parse_colors was deprecated (figuring out in which version this was changed and pyvista version should be set higher as this version in the requirements)
• Checking if there is still a dependency conflict for #719

None-critical issues but nice-to-have:

• Rename master to main
• GemPy currently returns an error, when the index of the provided input DataFrames is not consecutive or duplicated. This could be fixed with #762
• Add citation file
• Replace vtk function with pyvista function #760
• Clean up issues (what is still relevant)