[Therion] changing page size in atlas

[Stacho Mudrak]

Quoting Wookey <wookey at aleph1.co.uk>:
> Ah. OK. I did wonder. The thbook docs imply that you only need to set two
> of those variables, the other one will be calculated. I'l try again.

I am sorry, it is a BUG in therion. Currently, it is automatically  
calculated, when it is directly specified after export. E.g.

export atlas -layout-size 20 20 cm

I will try to fix it.

S.