Fix/default constructor initializer list

A default constructed FoamGrid should be a valid (but empty) grid. The list of entites was previously uninitialized but this data member is accessed in several interface functions.

The entity list is now properly initialized. Level 0 always exists, but is empty (empty entity lists) when the grid is default constructed. An empty level index set is also created for level 0.

Improves test by adding an empty grid test.

added 5 commits

• e424e8fb - [foamgrid] Initialize entity vector and level 0
• 2444a7ff - [test] Fix includes
• d2293f82 - [test] Add empty foamgrid to foamgrid test
• bd1abdf0 - [test] Fix compiler warnings
• ad0bc953 - [test] Add 1d-1d foamgrid to test

Compare with previous version

added 6 commits

• da3c2a69 - [foamgrid] Initialize entity vector and level 0
• b1aee8f5 - [test] Fix includes
• b7abc1b4 - [test] Add empty foamgrid to foamgrid test
• 35976896 - [test] Fix compiler warnings
• 2332538d - [test] Add 1d-1d foamgrid to test
• 03eada5d - Throw error if levelIndexSet of nonexisting level is requested

Compare with previous version

merged

mentioned in commit 30e46f88

mentioned in issue #13 (closed)