Inconsistency between trainInstancesDir and testInstancesDir

[MLopez-Ibanez]

By default

# trainInstancesDir = "./Instances/"
# testInstancesDir = ""

So not specifying any directory, like:

## File with a list of instances and (optionally) parameters.
## If empty or NULL, do not use a file.
trainInstancesFile = "instances-train.txt"

## File containing a list of test instances and optionally additional
## parameters for them.  If empty or NULL, do not use a file.
testInstancesFile = "instances-test.txt"

defaults to for train instances to Instances/train-instance-1 ...
but to ./test-instance-1 ... for test instances, which is unexpected.

# FIXME: Use NA as default value for testInstancesFile/testInstancesDir.
If testInstancesFile != NA and testInstancesDir == NA,

then make testInstancesDir = trainInstancesDir. Maybe give a warning when doing this.