Added functionality to throw errors when incorrect indentation used inside a section and when a standalone parameter is indented.

Conflicts:
	src/parser.c
	src/parser.h
	tests/testParser.c

First implementation of gpart motion

The changelog is rather big but the changes are rather straightforward:

 - The acceleration of the `gparts` is now called `a_grav` instead of just `a`. Makes grepping through the files much easier.
 - I have added the command-line options `-g` and `-G` for external- and self-gravity respectively. These build the correct policies into the engine. Separating both (at least for now) allows to work on the two aspect separately.
 - There are tests in critical places to make sure that linked pairs part-gpart are at the same position.
 - The `doinit()`, `dodrift()` and `dokick()` tasks have been updated to also move the gparts.
 
All the tests pass successfully. I am seeking your opinion on the implementation of the tasks. 

See merge request !135