Add search functionality to database

[derekjxtan]

Added the following functionality from to database class:

1. search for doctor with a given nric
2. search for patient with a given nric

closes #82

[codecov[bot]]

Codecov Report

Merging #83 (5c22645) into master (de6121b) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #83      +/-   ##
============================================
+ Coverage     77.54%   77.60%   +0.05%     
- Complexity      518      522       +4     
============================================
  Files            79       79              
  Lines          1550     1554       +4     
  Branches        156      156              
============================================
+ Hits           1202     1206       +4     
  Misses          313      313              
  Partials         35       35              

Files	Coverage Δ
src/main/java/seedu/address/model/Database.java	92.18% <100.00%> (+0.52%)	:arrow_up:

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