Default behaviour change to write ensembles to dense format when ensemble size > 1e6

[briochh]

This PR changes default behaviour to write ensembles to dense binary format when the number of obs or parameters exceeds 1e6.

This is consistent with changes coming in PEST++ and favours the dense binary format for more reliable and faster IO when the ensembles get big.

USERS BEWARE: Will add a '.bin' extension to the requested filename if nobs or npars exceeds 1e6. Happy to discuss alternative options -- like error raising, or agnostically propagating filename but this seems the safest and leas disruptive. Ensemble.to_binary() should now return the written filename.

[coveralls]

coverage: 79.088% (+0.2%) from 78.91%
when pulling ca340d2 on briochh:feat_densedefault
into b8d6033 on pypest:develop.