safely remove qudits from a register

[GiggleLiu]

• isseparable returns true if a quantum state is separable.
• join can be join two density matrices now,
• safe_remove! function can be used to remove ancilla qudits that separable from other qudits,

[codecov[bot]]

Codecov Report

Merging #420 (fead116) into master (2006fd0) will increase coverage by 0.03%. The diff coverage is 89.74%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   89.41%   89.44%   +0.03%     
==========================================
  Files          73       73              
  Lines        4476     4537      +61     
==========================================
+ Hits         4002     4058      +56     
- Misses        474      479       +5     

Impacted Files	Coverage Δ
lib/YaoBlocks/src/autodiff/chainrules_patch.jl	85.48% <50.00%> (+2.15%)	:arrow_up:
lib/YaoArrayRegister/src/density_matrix.jl	98.46% <100.00%> (+0.07%)	:arrow_up:
lib/YaoArrayRegister/src/operations.jl	84.61% <100.00%> (+2.33%)	:arrow_up:
lib/YaoArrayRegister/src/register.jl	95.78% <100.00%> (-2.30%)	:arrow_down:
lib/YaoBlocks/src/primitive/const_gate_tools.jl	93.75% <0.00%> (+2.20%)	:arrow_up:

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