Work on block assembler (FEniCS#63)

* Fix some block vector assembly issues.

* Fix l2g map for block vectors.

* Some nest matrix intialisation fixes.

Some bugs remain.

* Fix for block mat row indexing.

* Matrices with local sub-mats and bcs now seem to work . . . .

* Fix setting if diagonal for matrix entries with bcs.

* Some generalissation of a test.

* Work on assembling vectors into a local array.

* Fix stacking of sparsity patterns for non-square blocks.

* Clean up some debug output.

* Work on MattNest support.

* Get all matrix assembly cases working.

* Update block assembly tests so vector norm check doesn't require petsc4py-dev.

.gitignore

@@ -127,3 +127,5 @@ cmake.local
 build/
 
 .vscode
+*.egg-info
+.pytest_cache