Support strftime and strptime equivalent for formatting and parsing

[ChristopherRabotin]

For #182, this adds support for the computation of Day of Year (e.g. 059 for the 29th of February 2000). This also supports initialization of an Epoch provided the year, the Day of Year, and the time scale.

For #181, this adds support for custom formatting rules, as well as providing a number of standard formats like RFC2820. This also introduced the month name structure. This structure is used strictly for formatting and has no other use. It is therefore extremely limited in scope.

Notes on the implementation of the parsing: if the week day is provided in the format and the string to parse leads to an Epoch whose week day is different from the one provided, the parsing will return an error.

@gwbres I've added you as a reviewer since you've contributed a lot recently. I'll be self-reviewing the changes proposed here anyway, but a second pair of eyes is always useful.

Remaining TODOs:

• Parsing provided an EpochFormat and a string
• Convert the current from_str to the parsing method above (I'm unsure about this because the current from_str is quite well optimized so there may be some performance degradation if the switch is made).

[codecov-commenter]

Codecov Report

Base: 79.83% // Head: 77.67% // Decreases project coverage by -2.15% ⚠️

Coverage data is based on head (b1071b9) compared to base (09bfcf3).
Patch coverage: 69.43% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   79.83%   77.67%   -2.16%     
==========================================
  Files          10       14       +4     
  Lines        2886     3602     +716     
==========================================
+ Hits         2304     2798     +494     
- Misses        582      804     +222     

Impacted Files	Coverage Δ
src/deprecated.rs	0.00% <0.00%> (ø)
src/lib.rs	100.00% <ø> (+42.85%)	⬆️
src/month.rs	30.90% <30.90%> (ø)
src/errors.rs	37.50% <37.50%> (ø)
src/efmt/formatter.rs	50.00% <50.00%> (ø)
src/duration.rs	82.18% <63.38%> (-1.83%)	⬇️
src/parser.rs	77.22% <69.23%> (-2.55%)	⬇️
src/efmt/format.rs	79.55% <79.55%> (ø)
src/epoch.rs	87.67% <96.59%> (+0.15%)	⬆️
src/timescale.rs	94.49% <100.00%> (+1.56%)	⬆️
... and 4 more

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[gwbres]

Hello Chris,

@gwbres I've added you as a reviewer since you've contributed a lot recently. I'll be self-reviewing the changes proposed here anyway, but a second pair of eyes is always useful.

thanks! I'll test this new content when I have a little time.

Wow this is very good Chris. This will probably help simplify operations like here where I need to parse an Epoch from a custom format, and generate it a few lines above