Small changes in API

[ahojukka5]

• New function get_problem(problem, name) to fetch a problem from the analysis.
• Update docstrings.
• Connectivity can be given with tuple when creating new element.
• Be more information (@info) when calling functions to be more responsible.
• Preparations to separate topology and basis.
• Remove function to update properties with string pairs, it's quite an unpractical approach (we still should implement something like update_properties!(...) ).
• add_elements!, add_problems!, takes also varargs, so can also call add_elements!(p, e1, e2) instead of creating first iterable.
• update! can take list of elements as tuple.
• SparseMatrixCOO and SparseVectorCOO can be converted to full matrices / vectors using Matrix and Vector.
• Comparison of SparseMatrixCOO with matrix / vector using isapprox.
• add! returns nothing.

[coveralls]

Coverage increased (+0.2%) to 99.258% when pulling 5d5fdb007c43ddff844473d6c13828d7da3cd8c6 on release-0.2.1 into 5201c70c23eee966406c86548ff25ff8e5a35dd8 on master.