Introduce HeaderClass

[VOMAY]

Dear Michael,

I am looking into speeding up the RINEX3 processing. While reading the code, I saw that the header information handling is done quite confusing. I thought of an easier way to using a HeaderClass, which I want to implement first. I am convinced its more comprehensible and easier to maintain. I also took some ideas from @anand-satnav

What do you think about this solution? If you like it, I will continue to implement it, while also fixing the speed. I am willing to implement this also for rinex2 and nav, which should eventually shorten the code.

Note: The code does not run yet.

[lgtm-com]

This pull request introduces 4 alerts when merging e053e03 into 03998ad - view on LGTM.com

new alerts:

• 2 for Unused local variable
• 1 for 'import *' may pollute namespace
• 1 for Wrong number of arguments in a call

[lgtm-com]

This pull request introduces 2 alerts when merging 44107e1 into 03998ad - view on LGTM.com

new alerts:

• 1 for Unnecessary pass
• 1 for Unused import

[VOMAY]

I put in some more work and I like the result a lot. The code is more comprehensible and much faster. I managed to read a 40MB RINEX3 obs file in 4 seconds on a windows notebook.

I modified the coordinates, like I proposed in the past, to (const, prn, time). This makes data slicing easier for many applications.

[scivision]

Sorry for slow reply--thanks for thinking about this and the initial implementation!
I am also thinking in light of what you and others have said, to make georinex use Dict instead of xarray, because of the non-rectangular nature of GNSS data over hours, where satellites are going in and out of view, making so many NaNs in a rectangular xarray.DataArray