[SYSTEMML-949] Delete ANTLR tokens files during initialize phase
[SYSTEMML-949] Delete ANTLR tokens files during initialize phase

Delete ANTLR tokens files during initialize phase of maven lifecycle
so they can be regenerated during the generate-sources phase of maven
lifecycle that the antlr4-maven-plugin's antlr4 goal binds to.
Add maven-clean-plugin to lifecycle-mapping to remove m2e warning.

Closes #253.

gweidner/systemml,dhutchis/systemml,akchinSTC/systemml,dusenberrymw/systemml,dhutchis/systemml,nakul02/systemml,asurve/systemml,asurve/arvind-sysml2,asurve/incubator-systemml,deroneriksson/systemml,nakul02/incubator-systemml,dusenberrymw/systemml,sandeep-n/incubator-systemml,niketanpansare/systemml,deroneriksson/systemml,niketanpansare/systemml,nakul02/incubator-systemml,niketanpansare/systemml,apache/incubator-systemml,nakul02/systemml,gweidner/incubator-systemml,sandeep-n/incubator-systemml,dusenberrymw/incubator-systemml,niketanpansare/incubator-systemml,fschueler/incubator-systemml,deroneriksson/systemml,niketanpansare/incubator-systemml,fschueler/incubator-systemml,sandeep-n/incubator-systemml,nakul02/incubator-systemml,deroneriksson/incubator-systemml,gweidner/incubator-systemml,akchinSTC/systemml,deroneriksson/incubator-systemml,asurve/incubator-systemml,nakul02/systemml,dusenberrymw/systemml,akchinSTC/systemml,dusenberrymw/systemml,dusenberrymw/incubator-systemml,gweidner/systemml,nakul02/incubator-systemml,fschueler/incubator-systemml,niketanpansare/incubator-systemml,nakul02/incubator-systemml,deroneriksson/systemml,dusenberrymw/systemml,dusenberrymw/incubator-systemml,iyounus/incubator-systemml,deroneriksson/incubator-systemml,asurve/arvind-sysml2,nakul02/systemml,gweidner/incubator-systemml,deroneriksson/incubator-systemml,dhutchis/systemml,asurve/systemml,nakul02/incubator-systemml,dusenberrymw/systemml,dhutchis/systemml,iyounus/incubator-systemml,asurve/incubator-systemml,iyounus/incubator-systemml,deroneriksson/systemml,gweidner/incubator-systemml,akchinSTC/systemml,asurve/incubator-systemml,nakul02/systemml,deroneriksson/incubator-systemml,gweidner/systemml,apache/incubator-systemml,gweidner/incubator-systemml,akchinSTC/systemml,niketanpansare/systemml,akchinSTC/systemml,iyounus/incubator-systemml,dhutchis/systemml,asurve/arvind-sysml2,apache/incubator-systemml,asurve/arvind-sysml2,niketanpansare/systemml,iyounus/incubator-systemml,asurve/arvind-sysml2,iyounus/incubator-systemml,asurve/arvind-sysml2,asurve/systemml,asurve/incubator-systemml,gweidner/systemml,asurve/incubator-systemml,gweidner/incubator-systemml,dusenberrymw/incubator-systemml,apache/incubator-systemml,asurve/systemml,gweidner/systemml,niketanpansare/incubator-systemml,asurve/systemml,niketanpansare/systemml,deroneriksson/systemml,apache/incubator-systemml,gweidner/systemml,dusenberrymw/incubator-systemml,nakul02/systemml,sandeep-n/incubator-systemml,asurve/systemml,fschueler/incubator-systemml,apache/incubator-systemml,dusenberrymw/incubator-systemml,dhutchis/systemml,deroneriksson/incubator-systemml