Fix writing of Number=A and G INFO/FORMAT fields

[lennax]

Currently, the Reader stores field Numbers of A and G as negative integers, and the Writer doesn't convert them back to letters.

To make it easier to reverse the cast to integer, I changed the conversion method. Instead of converting using hard-coded if/elif, I've made a module-level dictionary for both conversion and reversal lookup. I'm using if/else because I've read that it's faster than try/except if the check is expected to fail over half the time, and I put if not first because it's more likely.

For writing the header fields, I switched to use str.format() (instead of C-style) and added a method that uses the conversion dictionary to switch the integers back to the correct letters.

In the third commit I removed the '.' if None str() _mapped to each line. The dictionary handles None to '.' for the Number field. As far as I can tell from the spec, only Number is allowed to have a value of '.' -- if this is incorrect this commit can be skipped.

[lennax]

Added another commit to write a None alt as '.'

Current logic is str(x) or '.' but str(None) evaluates to 'None' which is True.