add support for structures about bosl2

[adrianVmariano]

I wrote merged parse_pair() into struct_set, and also added is_struct. I was wondering if is_struct should check that there are no duplicated keywords, which might make it slow.

// Function: struct_set()
// Usage:
//    struct_set(struct, keyword, value, [grow])
//    struct_set(struct, [keyword1, value1, keyword2, value2, ...], [grow])
// Description:
//    Sets the keyword(s) in the structure to the specified value(s), returning a new updated structure.  If a keyword
//    exists its value is changed, otherwise the keyword is added to the structure.  If grow is set to false then
//    it is an error to set a keyword not already defined in the structure.  If you specify the same keyword twice
//    that is also an error.  If speed matters, use the first form with scalars rather than the list form: this is
//    about thirty times faster. 
// Arguments:
//    struct = input structure
//    keyword = keyword to set
//    value = value to set the keyword to
//    grow = Set to true to allow structure to grow, or false for new keywords to generate an error.  Default: true
function struct_set(struct, keyword, value=undef, grow=true) =
    let(aa=echo(struct=struct, keyword=keyword, value=value, grow=grow))
     !is_list(keyword) ?
        let(ind=search([keyword],struct,1,0)[0])
        (ind==[] ? assert(grow,str("Unknown keyword \"",keyword))
                   concat(struct, [[keyword,value]]) :
               list_set([ind], [[keyword,value]],struct)) :
               _parse_pairs(struct,keyword,grow);

function _parse_pairs(spec, input, grow=true, index=0, result=undef) =
     assert(len(input)%2==0,"Odd number of entries in [keyword,value] pair list")
     let( result = result==undef ? spec : result)
     index == len(input) ? result :  let(dd=echo(dd=spec, input, grow,index, result))
   _parse_pairs(spec,input,grow,index+2,struct_set(result, input[index], input[index+1],grow));

// Function is_struct()
// Usage:
//    is_struct(struct)
// Description: Returns true if the input has the form of a structure, false otherwise.  
function is_struct(struct, ind=0) =
     struct == [] ||
       let(dim = array_dim(struct))
       len(dim)==2 && dim[1]==2;