h4h5tools

Improve EOS2 conversion performance. Previously, one whole grid (or swath) structure was read whenever a field is visited, which gives very pure performance. This changelist remembers the last grid (or swath) structure to avoid re-read the whole structure again and again. If a field belongs to the same grid (or swath) that the previous field belongs to, the remembered thing is just reused.

Assuming that one grid (or swath) contains several fields and fields are traversed in DFS manner, a field and the subsequent field tend to belong to the same grid (or swath) in most times. This chnagelist exploits this fact.