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.
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
Bugfix