Change the code coverage to green. 

* removed duplicate code

* Removed unused code

* removed unmaintained code

* DCTI + DVTI refactored

* discrete fields refactored and tested

* fields are now tested quite well.

* Removed obsolete code not used anywhere

* Element descriptions to common dictionary

* size in global const dictionary also

* Added coverage to sparse tools and removed couple unused functions

* get nonzero rows from SparseMatrixCSC

* bugfix: extending element basis now working and tested

* Removed two unused functions from elements.jl

* removed useless function

* Useless conversion

* remove elasticity assembly using ForwardDiff because it's not used anywhere'

* Added basic testing for NURBS. Fixed bug in NSolid interpolation.

* removed unused functions

* Removed some debug stuff

* renamed file

* removed field assembly posthook, i think not good idea at all

* test for nnz(K) == 0 and automatic determination of dofs

* Testing that solver is throwing error if having problems with boundary assembly

* Removed some unused options. Refactoring.

* Moved solver non-related code to elements.jl

* Removed custom exception (no need)

* unneeded postprocess code

* More tests for NURBS elements.

* Removed unfinished .mail parser

* proper use of Logging package

* also read results

* renamed test file

* create_surface_elements accepts surface name in String now

* bugfix: remove zero rows from constraint matrix after manually removing dofs from some boundary assemblies.

* New test, displacement 3d patch test

* skip displacement field in surface element splitting if not defined

* test element splitting and linear surface elements, fails.

* Bugfix: Xdmf, not XDMF

* removed nonworking tests, requires bugfix

* abaqus_read_results is not working -> bug