thellier_gui: fix issue with empty er_site_name in magic_measurements.txt
thellier_gui: fix issue with empty er_site_name in magic_measurements.txt

thellier_gui.py: if er_site_name is an empty column (for example
synthetic specimens) then use “er_sample_name” instead of
“er_site_name” when reading site data from magic_measurements.txt .

lfairchild/PmagPy,Caoimhinmg/PmagPy,lfairchild/PmagPy,lfairchild/PmagPy,Caoimhinmg/PmagPy,Caoimhinmg/PmagPy