Cleanup ModelManager

[juliusgambe]

Closes #490

Cleanup Model Manager

New Methods

Customer

1. showAllCustomerList()
   May impact: CustomerEditCommand, CustomerListCommand, UserLoginCommand

2. getFilteredCustomerListSize()
   May impact: CustomerFind

3. isFilteredCustomerListEmpty()
   May impact: CustomerList

Delivery

1. showAllDeliveryList()
   May impact: DeliveryEditCommand, DeliveryListCommand

2. getFilteredDeliveryListSize()

3. isFilteredDeliveryListEmpty()

4. isSortedDeliveryListEmpty()

Other Changes

1. Extracted out transformers for LOD
2. Refractor guard statements to emphasise happy paths instead

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (497b56b) 83.97% compared to head (9b3fe30) 84.02%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #493      +/-   ##
============================================
+ Coverage     83.97%   84.02%   +0.04%     
- Complexity      977      984       +7     
============================================
  Files           121      121              
  Lines          2895     2904       +9     
  Branches        342      342              
============================================
+ Hits           2431     2440       +9     
  Misses          397      397              
  Partials         67       67              

Files	Coverage Δ
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	99.44% <100.00%> (+0.02%)	⬆️

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[jianyangg]

LGTM. Thanks for adding these. We have to remember to remove deprecated methods once everyone made their changes.