parse query on Select object creation

[lmiq]

This PR makes the use of the selection string @sel_str much faster, because the query is parsed on the construction of the object, and not on the application of the query to each atom. For example:

Before:

julia> struc = retrievepdb("4YC6")

julia> @btime collectatoms($struc, $(sel"name CA"));
  37.468 ms (605588 allocations: 16.10 MiB)

After:

julia> @btime collectatoms($struc, $(sel"name CA"));
  889.474 μs (41122 allocations: 2.06 MiB)

Also, the Select object has a custom show method:

julia> sel"name CA and resnum 1"
Select("name CA and resnum 1")

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.95%. Comparing base (a98a59d) to head (f1d622b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   94.93%   94.95%   +0.01%     
==========================================
  Files          14       14              
  Lines        1897     1901       +4     
==========================================
+ Hits         1801     1805       +4     
  Misses         96       96              

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[jgreener64]

Thanks a lot!