New PROPOSAL installation process. Change from gitsubmodules to conan.

.gitlab-ci.yml

.gitmodules

CMakeLists.txt

conanfile.txt

corsika/detail/modules/proposal/ContinuousProcess.inl

corsika/detail/modules/proposal/Interaction.inl

corsika/detail/modules/proposal/ProposalProcessBase.inl

corsika/detail/modules/pythia8/Decay.inl

corsika/detail/modules/pythia8/Interaction.inl

corsika/detail/modules/sibyll/Interaction.inl

corsika/modules/proposal/ContinuousProcess.hpp

corsika/modules/proposal/Interaction.hpp

corsika/modules/proposal/ProposalProcessBase.hpp

examples/vertical_EAS.cpp

modules/CMakeLists_PROPOSAL.txt

modules/proposal

tests/modules/testPythia8.cpp