Bug in MCMC calibration plots when more than one variable used for calibration

[albinwwells]

[albinwwells]

The plot_mcmc_chain function has a code block in a for loop where the pred_primes_key and pred_chain_key variables are overwritten such that subsequent iterations of the loop will fail to run. In the current code, this for loop only executes once since only one type of dataset (elevation change) is currently supported for calibration other than the mass balance. However, this fix makes the code more robust for development and future changes.