Closes #91 - Alternatives are not built prior to DSE

* alternatives are now checked and built, in case the alternatives
  dimension is activated in DSE

src/main/scala/tapasco/dse/Alternatives.scala

@@ -58,8 +58,19 @@ object Alternatives {
     * @param target Target Architecture and Platform
     * @return Set of alternative names for kernel on Target **/
   def alternatives(kernel: String, target: Target)(implicit cfg: Configuration): Set[String] =
-    //idForName(kernel) map (namesForId(_, target)) getOrElse(Set())
     idForName(kernel) map (namesForId(_)) getOrElse(Set())
+ 
+  /** Returns alternative kernels for given kernel.
+   *  @param kernel Kernel to find alternatives to.
+   *  @return Set of alternative Kernels. **/
+  def alternatives(kernel: Kernel)(implicit cfg: Configuration): Set[Kernel] =
+    FileAssetManager.entities.kernels.filter(_.id equals kernel.id)
+
+  /** Returns alternative kernels for given kernel name.
+   *  @param name Name of Kernel; this may be used to find alternatives to an Verilog/VHDL core.
+   *  @return Set of alternative Kernels. **/
+  def alternatives(name: String)(implicit cfg: Configuration): Set[Kernel] =
+    FileAssetManager.entities.kernels.filter(k => idForName(name) map (_ equals k.id) getOrElse false)
 
  /** Returns alternative compositions on given Target.
    * @param bd Original Composition

src/main/scala/tapasco/jobs/executors/DesignSpaceExploration.scala

@@ -23,6 +23,7 @@
  **/
 package de.tu_darmstadt.cs.esa.tapasco.jobs.executors
 import  de.tu_darmstadt.cs.esa.tapasco.base._
+import  de.tu_darmstadt.cs.esa.tapasco.dse._
 import  de.tu_darmstadt.cs.esa.tapasco.filemgmt._
 import  de.tu_darmstadt.cs.esa.tapasco.task._
 import  de.tu_darmstadt.cs.esa.tapasco.jobs._
@@ -42,12 +43,18 @@ private object DesignSpaceExploration extends Executor[DesignSpaceExplorationJob
     val kernels = job.initialComposition.composition map (_.kernel) toSet
 
     logger.debug("kernels found in composition: {}", kernels)
+    logger.debug("alternative kernels: {}", kernels map (Alternatives.alternatives _))
 
-    val missing = for {
+    val missing = (for {
       k <- kernels
       t <- job.targets
       if FileAssetManager.entities.core(k, t).isEmpty
-    } yield (k, t)
+    } yield (k, t)) ++ (for {
+      k <- kernels
+      t <- job.targets
+      a <- Alternatives.alternatives(k)
+      if job.dimensions.alternatives && FileAssetManager.entities.core(a.name, t).isEmpty
+    } yield (a.name, t))
 
     if (missing.nonEmpty) {
       logger.info("need to synthesize the following cores first: {}",