Fix rounding error in depreciation value using decimal arithmetic

[ankurdave]

What changes were proposed in this pull request?

This plugin currently converts start and end values from Decimal to float before doing any calculations. float is appropriate for generating the depreciation curves, but not for calculating the depreciation values that will go into the ledger. As a result, for many choices of start value, floating-point error causes this plugin to produce unbalanced transactions like the following:

2020-04-30 * "Test-auto_depreciation:Nikon"
  Assets:Fixed-Assets                               -3 LENS {999.95 CNY, 2020-03-31, "Nikon"}
  Assets:Fixed-Assets                                3 LENS {559 CNY, 2020-04-30, "Nikon"}
  Expenses:Depreciation  1322.850000000000136424205266 CNY

The fix is to keep the start and end values as Decimals except when generating the depreciation curves.

How was this patch tested?

Added an end-to-end test called test_rounding_errors() that previously failed and now passes.

[hktkzyx]

Thanks for your PR so much! I will merge your commit very soon.