Reject a TLE whose epoch field contains a letter

[Wahaj18]

Fixes #427.

Checksum_Good() counts any character other than a digit or - as 0, so a letter that replaces a 0 in the epoch field (columns 19-32 of line 1) leaves the checksum valid. Good_Elements() accepted such a set, and Convert_Satellite_Data() then read the epoch only up to the letter. With the example from the issue the epoch moves 14 min 16 s early without any error.

This adds a check to Good_Elements() that the epoch field is a well-formed number:

• a two digit year,
• a three digit day of year, whose two leading digits may be padded with spaces (the case Convert_Satellite_Data() already handles),
• a fraction of day made of digits, followed by nothing but trailing spaces.

A set that fails the check is rejected the same way as one with a bad checksum, so Get_Next_Tle_Set() returns -2 and the TLE update and satellite loading code skip it.

Testing

• test-001 now builds a copy of its element set with O in place of a 0 in the epoch and fails if Good_Elements() accepts it, and also checks that a space-padded day of year is still accepted. It fails without the change and passes with it.
• ./autogen.sh && make && make check pass; test-001 and test-002 pass.
• All 15941 element sets in the current CelesTrak active list are still accepted by Get_Next_Tle_Set(), before and after the change.

[csete]

Thanks, it's merged