incorrect error case handling for OSGridError

[Chris-Papaioannou]

The following code incorrectly results in an error flag:

from PyBNG import PyBNG
PyBNG(easting = 328993, northing = 97946).get_latlon()

Error code:

File "{USER}\AppData\Roaming\Python\Python39\site-packages\OSGridConverter\grid.py", line 71, in init
factor=pow(10,5)
OSGridConverter.base.OSGridError: OSGridConverter error: Invalid grid reference

It seems any BNG grid ref with 6 digits in one coordinate and 5 digits in the other is rejected.

[ThunderStruct]

This isn't a bug, BNG coordinates should have an even-numbered set of digits (from 2 to 10, depending on the quadrant size). Here's a reference that should help!

I'll leave this issue open for now nonetheless as a reminder to add better error-handling/clarify why a given grid reference is invalid. Good luck!