What is the motivation for using a class method here?  My guess would be that you are effectively using the `iers_table` class attribute as a cache to avoid re-loading the IERS table.  But this strategy can have some confusing consequences, like:
```
iers1 = IERS(iers_file1)
iers2 = IERS(iers_file2)
# Ooops, iers2 silently points to the data in iers_file1
```
So instead you could make these normal instance methods and use a module-level cache of the data tables which is keyed on the file name.