Fix compiler warnings

[TobiasDuswald]

This PR fixes the following compiler warnings:

[493/533] Building CXX object CMakeFiles/biodynamo-unit-tests.dir/test/unit/core/agent/agent_test.cc.o
/Users/rdm/biodynamo/test/unit/core/agent/agent_test.cc:306:8: warning: unused variable 'nptr' [-Wunused-variable]
  auto nptr = neighbor->GetAgentPtr<Cell>();
       ^
/Users/rdm/biodynamo/test/unit/core/agent/agent_test.cc:350:8: warning: unused variable 'aptr1' [-Wunused-variable]
  auto aptr1 = agent1->GetAgentPtr<Cell>();
       ^
/Users/rdm/biodynamo/test/unit/core/agent/agent_test.cc:356:8: warning: unused variable 'aptr2' [-Wunused-variable]
  auto aptr2 = agent2->GetAgentPtr<Cell>();
       ^
/Users/rdm/biodynamo/test/unit/core/agent/agent_test.cc:399:8: warning: unused variable 'naptr' [-Wunused-variable]
  auto naptr = new_agent->GetAgentPtr<Cell>();
       ^
/Users/rdm/biodynamo/test/unit/core/agent/agent_test.cc:430:8: warning: unused variable 'aptr1' [-Wunused-variable]
  auto aptr1 = agent1->GetAgentPtr<Cell>();
       ^
/Users/rdm/biodynamo/test/unit/core/agent/agent_test.cc:436:8: warning: unused variable 'aptr2' [-Wunused-variable]
  auto aptr2 = agent2->GetAgentPtr<Cell>();
       ^
6 warnings generated.
[509/533] Building CXX object CMakeFiles/biodynamo-unit-tests.dir/libbiodynamo-unit-tests_dict.cc.o
In file included from /Users/rdm/biodynamo/build/libbiodynamo-unit-tests_dict.cc:49:
/Users/rdm/biodynamo/test/unit/core/resource_manager_test.h:462:21: warning: variable 'd' set but not used [-Wunused-but-set-variable]
    volatile double d = 0;
                    ^
1 warning generated.
[529/533] Building CXX object CMakeFiles/biodynamo-unit-tests.dir/test/unit/core/resource_manager_test.cc.o
In file included from /Users/rdm/biodynamo/test/unit/core/resource_manager_test.cc:16:
/Users/rdm/biodynamo/test/unit/core/resource_manager_test.h:462:21: warning: variable 'd' set but not used [-Wunused-but-set-variable]
    volatile double d = 0;
                    ^
1 warning generated.
[533/533] Linking CXX executable bin/biodynamo-unit-tests

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