generic sqw_op

[abuts]

Horace have reach set of operations to work with sqw objects. Plus, minus, sqw_eval replicate, etc allow one to modell various processes occuring with sqw objects and analyze them extracting, adding and mixing various modells with each other.

Recently these operations have been extended to filebased sqw objects. The only issue with filebacked objects still remains is that an operation over whole sqw objects takes say, abot 5-10 minutes and if you do 3 operations (e.g. model background (sqw_eval), extract model from sqw object, (minus) and mask negative values (mask) it will take 30 minutes to do them all.

The purpose of this ticket is to write overload to Page_op_sqw_eval which would allow user to write custom function which allows user to do all these operation in one go, so instead of doing filebacked operations three times, the user would apply all desired operations in one go.

Looks like this ticket would also resolve Re #1364 and touches Re #1372