Reduce number of ufc::form functions

[Martin Sandve Alnæs]

Along one dimension, ufc::form has a number of functions for each integral type:

    /// Return the number of cell domains
    virtual std::size_t num_cell_domains() const = 0;

    /// Return whether form has any cell integrals
    virtual bool has_cell_integrals() const = 0;

    /// Create a new cell integral on sub domain i
    virtual cell_integral* create_cell_integral(std::size_t i) const = 0;

    /// Create a new cell integral on everywhere else
    virtual cell_integral* create_default_cell_integral() const = 0;

This can be reduced to:

    /// Create new cell integrals, default and on all sub domains
    virtual std::pair<cell_integral*, std::vector<cell_integral*>> create_cell_integrals() const = 0;

or something similar, whatever is most convenient in dolfin.

Along another dimension, there are many integral types: cell, exterior_facet, interior_facet, point, custom. These can be made into enums or we can even use strings for full flexibility because these functions are only called rarely. Then we get:

    /// Create new integrals of given type, default and on all sub domains
    virtual std::pair<integral*, std::vector<integral*>> create_integrals(std::string type) const = 0;

instead of the 20 functions we have today. We might want to keep "has_integrals" in a generic form for convenience in dolfin:

    /// Has integrals of given type
    virtual bool has_integrals(std::string type) const = 0;

All details subject to whatever's convenient in dolfin code that uses this, these things are easy to generate however we want from ffc.

[Martin Sandve Alnæs]

The backside of doing this would be the need to do dynamic_cast<cell_integral*> etc. a few places in dolfin. Is that an ok tradeoff? I think it can be limited to UFC.cpp.