conf/expr: added repr for literal
Revert "conf/expr: concatenate list results to single string"

This reverts commit c55b1339301bfe7909bab96b9b6cda6d550ffd87.

this must not be done as the "ExpressionList" object will encapsulate
the expressions, and not a python list.
Revert "conf/variable: added exception for non-list assignment expressions"

This reverts commit 2861c63a79eb99b216a215a3df03028bfa45f34c.
builder: added logger imports
conf/assignment: added logger import
conf/variable: fixed evaluating the expressionlist of assignments
util/datatypes: fixed removal of nonexistant elements
tests: fixed util.path.parent_folder() call
tests: indexing and creating source list implemented
conf/assignment: expressionlist representation actually used now
tests: using absolute used sources references
tests: created dummy variables for being used in builder