relax strictness of the section handling and fix/work around a few bugs

We now do not require the sections to be correctly ordered and the
presence of the mandatory sections since even the unit tests did not
always specify all mandatory sections, which lead to a section
containing the rest of the deck if one of the expected next sections
was not specified.

also it seems like the DeckKeyword::getDeckIndex() does not correctly
work in some situations which lead

assert(deck->getKeyword(i)->name() == startKeyword);

to fail in the Section::populateKeywords() method. now the deck is
always sequentially traversed to find the position of a section's
start keyword. (This is necessary anyway if one wants to make sure
that the deck does not specify the same section more than once, a
feature which this patch also adds.)
considerably extend the unit test for sections

for this, an Section::isDeckValid() method is introduced which checks
that a given deck is valid when it comes to the sections
(i.e. presence of mandatory sections and section ordering)
Merge pull request #283 from andlaus/fix_section_handling

relax strictness of the section handling and fix/work around a few bugs