add challenge iterate macros

[gmanchon]

add macros to deal with the challenge context (cloned directory, data directory, and optionally other variables such as bootcamp - would a home make sense to address the prefect issue with ~ vs $HOME ? )

macros :

• # $ITE_CHALLENGE_PATH : cloned challenge path - vs a Kitt provided macro ?
• # $ITE_HOME : ~ vs $HOME
• # $ITE_DATA_PATH : # $ITE_HOME/$BOOTCAMP

exemple in notebooks/Lecture-livecode-Cloud-Training.ipynb :

"cd data-lecture-cloud-training\n",

replaced by a more stable (no need to worry if the instructions are correct until we have a test) :

"cd # $ITE_CHALLENGE_PATH\n",