Adding type parameters to PropertyComputationResult

[errt]

Taken from Bitbucket#142:

In FPCF we can compute various properties for a single entity. The analyses do so by computing a PropertyComputationResult. However, there is no type guarantee that the analyses always returns the intended result, e.g. an escape analysis should compute a Result that captures an escape property and not a purity property. The scenario becomes more realistic when it comes to wrapper properties, e.g. representing aggregated results. In cases where different entities/properties should be computed it would be still possible to use Entity/Property as type parameter

Stronger type guarantees would be still desirable, but are way more complex to achieve in the current setting.