Add special product variables for caching of byte offsets of top-level array elements

[svniemeijer]

To improve the reading performance of ASCII data and in general of 'array of record' style products (e.g. binary Level 0 data and GOME-2 L1 data) we should find a way to have the byte offsets of top-level array elements cached.

The idea is to do this using product variables. The problem with product variables in its current form is that it 1) requires the length of the variable to be decided before initialization and 2) (requiring a second pass through the product) a full initialization of the variable in one operation to assign all values of the product variable.

We could improve the performance by allowing these two steps to be performed using lazy initialization:

• allow product variables to start with length 0 and automatically resize the product variable array when higher indexed values are read
• allowing element-wise initialization (setting the index variable i to the array element that needs to be initialized)
• use a special 'missing value' (e.g. -1) that will trigger the initialization of that specific product variable array element

Next to this, we then also need to introduce an 'offset expression' for array types (similar to the 'offset expression' for record fields).