condition extractor -- dumping partial dumps

[tregua87]

Make the extractor able to dump function constraints right after the analysis terminates.

Add an additional flag to the tool (e.g., --partial_dump ??).

Probably add something like

if (OutputType == OutType::txt) {
    FunctionConditionsSet::storeIntoTextFile(
        fun_cond_set, OutputFile, verbose >= Verbosity::v1);
} else if (OutputType == OutType::json) {
    FunctionConditionsSet::storeIntoJsonFile(
        fun_cond_set, OutputFile, verbose >= Verbosity::v1);
} else if (OutputType == OutType::stdo) {
    SVFUtil::outs() << fun_cond_set.toString(verbose >= Verbosity::v1);
}

at around this line.

Then, add a cleaning method for FunctionConditionSet (here)

Partial results should be stored in a dedicated folder (another flag? a tmp folder?) and then grouped together through a dedicated tool (e.g., a python script to be invoked afterward).