Add a statistics box

[YangYue128-helen]

Now you can view the number of done/ upcoming appointments for today/this week in the UI.

[codecov-io]

Codecov Report

Merging #103 into master will increase coverage by 0.35%.
The diff coverage is 67.30%.

@@             Coverage Diff              @@
##             master     #103      +/-   ##
============================================
+ Coverage     75.83%   76.19%   +0.35%     
- Complexity      695      726      +31     
============================================
  Files           101      103       +2     
  Lines          2032     2096      +64     
  Branches        236      244       +8     
============================================
+ Hits           1541     1597      +56     
+ Misses          433      426       -7     
- Partials         58       73      +15     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../main/java/seedu/address/ui/StatisticsDisplay.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...ava/seedu/address/model/AppointmentStatistics.java	83.33% <83.33%> (ø)	8.00 <8.00> (?)
...main/java/seedu/address/model/AppointmentBook.java	93.47% <92.30%> (-0.47%)	18.00 <3.00> (+3.00)	⬇️
...a/seedu/address/model/appointment/Appointment.java	94.11% <100.00%> (+0.17%)	37.00 <1.00> (+1.00)
...ain/java/seedu/address/model/appointment/Date.java	95.83% <100.00%> (+1.38%)	16.00 <3.00> (+3.00)
...n/java/seedu/address/commons/core/index/Index.java	61.53% <0.00%> (-38.47%)	12.00% <0.00%> (+2.00%)	⬇️
...rc/main/java/seedu/address/model/patient/Nric.java	66.66% <0.00%> (-13.34%)	7.00% <0.00%> (+1.00%)	⬇️
... and 5 more

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[xz0127]

Other than the comments mentioned, the rest LGTM!! 👍

[JinHao-L]

LGTM! 👍