Rp2

* fixed a bug in timestamp check: in and out lots were not allowed to have the same timestamp, but in certain scenarios it could happen (e.g. high-frequency trading)
* fixed a subtle corner-case bug in LIFO, which caused the first gain/loss pair to be incorrect: the minimal input that reproduced the bug is now captured in one of the tests (test_data2.ods, sheet B1). This ensures there will not be a regression.
* disable a link check in user faq: the link was failing the check (incorrectly) on Github actions

* added new LIFO tests
* minor fixes

* added accounting-method programmable plugin infrastructure: by subclassing AbstractAccountingMethod it's now possible to add support for a new accounting method (previously only FIFO was hard-coded in)
* added accounting-method plugins: FIFO, LIFO
* added accounting-method-specific tests, plus additional tests of internal classes (both new and old)
* Added Legend sheet to tax_report_us output
* updated documentation to reflect changes in this version
* minor fixes

* added country-specific programmable plugin infrastructure: by subclassing AbstractCountry it's now possible to add support for a new country (currently only US is supported)
* abstracted out currency from APIs and code in general: references to "usd" have been changed to "fiat"
* updated documentation to reflect changes in this version

* Verified software is up to date for FY 2021
* Wrote several answers in User FAQ document
* Minor fixes

* Added several new tests (from/to_year command line option variations and new transaction types)
* Reworked time filter logic to fix two bugs related to -f and -t options:
* with time filtering on, running sums reflected only filtered transactions, instead of all transactions (in other words, running sums should not be affected by filtering)
* with time filtering on, number of fractions reflected also fractions with year higher than time filter
* with time filtering on, sold lot percentages would be shown incorrectly in some cases
* Updated documentation: various fixes and improvements