🐛 Reading of float variables in mfile.py

[chris-ashe]

Describe the bug

In plot_proc.py when trying to import from an MFILE using mfile_data.data the following error occurs:

Can't parse value in MFILE: ['helium']
Traceback (most recent call last):
  File "/home/PROCESS/process/io/mfile.py", line 361, in sort_value
    return float(value_words[0])
ValueError: could not convert string to float: 'helium'

This was speicfically the case when importing i_fw_coolant_type as such:

i_fw_coolant_type = mfile_data["i_fw_coolant_type"].get_scan(scan)