Added support for tetrads to OSRef.php.

[stevegoddard]

Updated OSRef.php to accept tetrads and convert them to eastings / northings.
Updated OSRef.php to allow export to tetrad (toGridReference method).
The tetrad grid letters go in a different orientation to the main OS grid (start bottom left and run vertically). They also contain the letter I and exclude the letter O.
A lot of wildlife data is recorded at tetrad resolution so hopefully this will be useful functionality.

[dvdoug]

@stevegoddard I'm happy to have this functionality added to the library, but since Tetrads seem to be not be an official OS referencing system, I think it would be clearer if it were to have a dedicated OSRef::fromTetrad method? What do you think?

[stevegoddard]

@stevegoddard I'm happy to have this functionality added to the library, but since Tetrads seem to be not be an official OS referencing system, I think it would be clearer if it were to have a dedicated OSRef::fromTetrad method? What do you think?

@dvdoug I can see your point. This would mean that anyone using the package would need to check the length of the grid ref before calling the correct method. Having the package check the length itself seems neater and easier. Up to you. If you want me to move it to it's own method I'll happily do that.

[dvdoug]

Since a Tetrad uses a different grid direction, this is something people would need to know anyway though? Or am I misunderstanding?

[stevegoddard]

Since a Tetrad uses a different grid direction, this is something people would need to know anyway though? Or am I misunderstanding?

We get data (wildlife sightings) in all different resolutions - often mixed together. So records for 1km grid squares (i.e. SO2242) and 2km (tetrad) squares (i.e. SO24G). Any grid ref 5 characters long (provided it's not an error) is a tetrad. So for us it's just simpler if the system checks the length and does the conversion. Just seems simpler - if it's 5 chars it must be a tetrad.