[GNC] Multi-split CSV Import

[flywire]

Geert, thank you. With that pretty simple explanation I completely
understand it.

I've rolled all that together ordering the split description lines the same
as the input form and added an example based on the Tutorial and Concepts
Guide. David T, I'm with you on contributing but iirc the development
environment to contribute to the docs is all but unachievable on Windows.
I'll leave this as a suggestion.

==========

* Multi-split: Selecting this check box allows the splits for a single
transaction to be defined on consecutive lines within the file with each
line defining a single split. If not selected each line is assumed to
contain the information for a single transaction including one or two
splits;

Multi-split is a gnucash specific variant of a csv file to support easier
data preparation.

Each line should define one split via:
- transaction-specific fields (date and description as bare minimum) *and*
- split specific fields (account and deposit/withdrawal as bare minimum).

There is a convenience feature built-in so you don't have to repeat the
transaction fields if the split belongs to the same transaction as the
previous one (though you can repeat them if that makes csv preparation
easier).

Sample Multi-split.csv

Date,Description,Deposit,Account
05/03/2006,Grocery Store,-45.21,Assets:Checking
,,45.21,Expenses:Groceries
14/03/2006,Employers R Us,670.00,Assets:Checking
,,180.00,Expenses:Taxes:Federal
,,90.00,Expenses:Taxes:Medicare
,,60.00,Expenses:Taxes:Social Security
,,-1000.00,Income:Salary