Incorrect handling of metastable-resolved ADF15 files

[vsnever]

At the moment, the metastable-resolved photon emissivity coefficients are not supported in Cherab. However, an attempt to install the PEC from a metastable-resolved ADF15 file does not throw an error and installs the PEC for the highest metastable index neglecting the PECs for other indeces. The correct behaviour would be detecting the metastable-resolved file on parsing and throwing an error.