read only mandatory extension keyword at start #22
Description
When parsing an HDU, an idea could be to only read the mandatory keyword with respect to an extension and stop the reading.
If the user is not interested on that HDU we will just have to loop over the remaining card searching for the END card (without parsing the cards) and jump the whole data block as usual.
If the user ask for the header, we parse the cards
In other words, this optimization avoids the cost of parsing the non mandatory cards when the user wants to skip the HDU.