Implement conditional mapping to RDF

[nvnikolov]

Add explicit support in the UI for conditional mappings. Conditional mappings will apply to sub-graphs or individual nodes.
Conditions: predicates and conditional operators (<, >, =, !=, etc.) and can be used to check values of a column of the current row. We should support the following outcomes:

• [] optional graphs - if the sub-graph fails to map for any reason, then it is not mapped to RDF (the rest of the mapping can still be executed)
• [] optional node mappings - map node differently (to one value/literal or another) depending on a condition
• [] optional property mappings - map properties differently (to one property or another) depending on a condition
• [] do not map triple on condition (this may overlap with optional graphs)
• [] choose between two/more mappings based on a condition