Improve data extraction in the RELAP5 plugin

[zhieejhia93]

Currently the RELAP5 plugin converts the plotfl text file into a CSV file and stores the selected data. This process could be problematic for large cases as the conversion process is time and memory consuming.

With the improvement, the plotfl file will no longer need to be converted to CSV. Instead, it will be read directly using PyPost which is a Python library from SNAP that can directly read the plotfl file. The new approach is significantly more efficient and is particularly suitable for large simulation cases.

Given that access to SNAP is restricted, the current plotfl-to-CSV conversion approach will NOT be removed for users without access to SNAP (or PyPost).