ILDG read/write needs an overhaul

[clarkedavida]

It is not written in a way that is general, readable, well-organized, or in some cases correct. For example:

• There are parts which rather pertain to LIME packing generally and not ILDG specifically. For example there is an 8-bit message begin/end flag which showed up only in a lime_record function. These 8-bit flags are really rather properties of LIME record headers, and so it would make more sense to have a LIME class of which these are attributes. The ILDG writer/reader should then inherit from this LIME class.
• There are very few comments, which makes it challenging to read for someone who doesn't know anything about LIME.
• It will read in at least some kinds of invalid LIME file without error. For example these message begin/end flags are never checked.
• It is not even clear to me how to set them correctly.
• It will read in many invalid ILDG files without error. As far as I can tell, none of the keywords necessary for ILDG are ever checked.

Maybe a better approach would be to try to port some already existing, stable LIME code into here. One could use e.g. https://github.com/usqcd-software/c-lime, but that may be a bit overkill. But at least it would be general and correct. I would recommend to then test our results against working LIME packers.

• For now, I am just going to get things "working", i.e. get our code to read in ILDG configs without checking their LIME layout, and to write out ILDG files that a LIME packer will not complain about. I will also add enough documentation that the next person can figure out how to implement it better.

This is not a small task. In my view it's really best suited for someone who has quite a bit of time and advanced C++ skill.

[clarkedavida]

Alright I did my share. The rest is up to someone else.

[clarkedavida]

this is also related to #136