CodeNRoll: add option to "read.simple.tsv()" to read only if its not in the mem space

[vertesy]

read.simple.tsv( ... , ReadOnlyIfVariableNotExist =  F)

if(ReadOnlyIfVariableNotExist){
exists(ReadOnlyIfVariableNotExist)
}

 # VariableExisting If not FALSE, it only reads in the file if a variable with the same name does NOT EXIST in the memory.

## usage:
read.simple.tsv( ... , ReadOnlyIfVariableNotExist =  "ExpressionData")