Support new OPTIONAL-related operators in cost model

[hartig]

PRs #213 and #214 have added new logical and new physical operators to add support for OPTIONAL. The implementation of the cost model (see the Java package se.liu.ida.hefquin.engine.queryproc.impl.optimizer.costmodel) does not yet consider these new operators. As a consequence, query plans that contain such an operator cause exceptions during cost estimation. For instance, the class CFRNumberOfTermsShippedInRequests throws an IllegalArgumentException in its initiateCostEstimation function if the root operator of the given plan is such an operator.

The TODO is to extend the implementation of the cost model to also support the new operators.

[hartig]

Another new operator (PhysicalOpParallelMultiLeftJoin) was added by PR #218.