Re-organize to reflect JuMP requirements

[odow]

This PR makes a number of changes to reflect functionality required by jump-dev/JuMP.jl#2114.

In particular, models now implement read! and write on io streams, and we provide a new_model function which takes a FileFormat enum and an (optional) filename string, and returns a new model object.

I've also removed the ability to specify a compression scheme when reading and writing to file in favor of AutomaticCompression. The reason being that users with different schemes should just use read!/write on the compressed io stream.

[codecov-io]

Codecov Report

Merging #100 into master will increase coverage by 0.01%.
The diff coverage is 96.51%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   96.17%   96.19%   +0.01%     
==========================================
  Files           9        9              
  Lines        1256     1261       +5     
==========================================
+ Hits         1208     1213       +5     
  Misses         48       48

Impacted Files	Coverage Δ
src/MPS/MPS.jl	99.23% <ø> (ø)	⬆️
src/MOF/read.jl	92.98% <ø> (ø)	⬆️
src/CBF/CBF.jl	99.23% <ø> (ø)	⬆️
src/MOF/write.jl	67.77% <ø> (ø)	⬆️
src/LP/LP.jl	97.64% <100%> (ø)	⬆️
src/MOF/MOF.jl	100% <100%> (ø)	⬆️
src/MathOptFormat.jl	100% <100%> (+5.08%)	⬆️
src/utils.jl	95.45% <95.45%> (ø)

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