Summary of changes

Added optional arguments: --fromDate and --toDate
- which specifies to download analysis created within (inclusive) the date range.
- validates the date upfront before
  - check if valid
  - converts to unix timestamp.
    - default values of fromDate and toDate are 0 (unix epoch) and current unix timestamp, respectively.
  For example:
```
irida-staramr-results -p 1 --fromDate 2021-01-04  --toDate 2021-03-18 -u irida-user -pw pasword1 -o out.xlsx
```
  This will export results from project 1, only exporting those results from analysis ran between January 4, 2021 and March 18, 2020, inclusive.
Added logging warning message if no analysis to be downloaded. No directory and files are created.

WARNING:root:No completed amr analysis submission created from [2021-04-06] to [2021-04-07]. Exiting..
Added time conversions functions
More unit tests

phac-nml / irida-staramr-results