Fixe cache level in Ingest objects

[pomadchin]

Overview

Without this fix in some cases we can be get into the situation where this cache level (IN_MEMORY) would be assigned for the RDD, making impossible to set the desired cache level from the parameters, causing the following exception:

[info]   java.lang.UnsupportedOperationException: Cannot change storage level of an RDD after it was already assigned a level
[info]   at org.apache.spark.rdd.RDD.persist(RDD.scala:170)
[info]   at org.apache.spark.rdd.RDD.persist(RDD.scala:195)
[info]   at geotrellis.spark.ingest.Ingest$.apply(Ingest.scala:96)