Add radioactive decay plugin

Add radioactive decay plugin.
Add postprocess plugin to output change in average heating rate.
Add function in simulator_access to get the pointer of heating model.
Bug fix

Add missing header
Bug fix
Change postprocess

Change file and class names to internal heating.
Add visualization plugin.
Change internal_heating.h

Bugfix on class names
Put seperate heating rate on crust and mantle
Bugfixes
Bugfixes
Bugfix
Bugfix

Missing conditions on composition loop.
Add postprocessor viscous_dissipation_statistics
Revise documentation
Minor changes

Revise documents
Add test prm file
Bugfix
Revise test file
Revise test file
Reviese test file
Revise test
Merge remote-tracking branch 'upstream/master' into Time_dependent_radioactive_heating
Minor changes

Fix typos and naming