fix an inexact error

[putianyi889]

This bug was introduced by the overhaul of Jacobi connection #145 , which basically replaced ≈ with ==.

julia> Integer(1.9-0.9)
ERROR: InexactError: Int64(0.9999999999999999)
Stacktrace:
 [1] Int64
   @ .\float.jl:994 [inlined]
 [2] Integer(x::Float64)
   @ Core .\boot.jl:926
 [3] top-level scope
   @ REPL[5]:1

also simplified some codes, particularly those introduced by #145 .

Edit: #211 conflicts with this... Apart from other code simplifications, isapproxinteger requires multiple computation of k. This PR reduces the computation to only once and also let the lower level handle the errors.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.01%. Comparing base (833245b) to head (cf2ec38).

Files with missing lines	Patch %	Lines
src/classical/jacobi.jl	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
- Coverage   92.14%   92.01%   -0.13%     
==========================================
  Files          18       18              
  Lines        1858     1841      -17     
==========================================
- Hits         1712     1694      -18     
- Misses        146      147       +1     

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