Add consts for Excel max sheet size and @assert them in writetable!

[nathanrboyer]

This fixes #246.

I found these limits hardcoded in other locations besides write.jl, so I replaced them with the const variables everywhere they appear.

MWE:

julia> using XLSX, DataFrames

julia> file = normpath(homedir(), "Downloads/test.xlsx");

julia> df = DataFrame(Symbol.(1:16_385) .=> "data")
1×16385 DataFrame
 Row │ 1       2       3       4       5       6       7       8       9       10      11  ⋯
     │ String  String  String  String  String  String  String  String  String  String  Str ⋯
─────┼──────────────────────────────────────────────────────────────────────────────────────
   1 │ data    data    data    data    data    data    data    data    data    data    dat ⋯

julia> df2 = DataFrame(x=1:1_048_576)
1048576×1 DataFrame
     Row │ x       
         │ Int64
─────────┼─────────
       1 │       1
       2 │       2
       3 │       3
    ⋮    │    ⋮

julia> XLSX.writetable(file, df, overwrite=true)
ERROR: AssertionError: `data` contains 16385 columns, but Excel only supports up to 16384; must reduce `data` size

julia> XLSX.writetable(file, df2, overwrite=true)
ERROR: AssertionError: `data` contains 1048576 rows, but Excel only supports up to 1048575; must reduce `data` size

Note that df2 needs to fail at row_count==1_048_576 rows because the column header puts it over the limit.

I would need help formalizing this MWE into tests if that is required.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (063b2e2) 94.76% compared to head (6d6df49) 94.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #247   +/-   ##
=======================================
  Coverage   94.76%   94.76%           
=======================================
  Files          15       15           
  Lines        2005     2007    +2     
=======================================
+ Hits         1900     1902    +2     
  Misses        105      105           

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[felipenoris]

looks good. thanks!