Update a subset of data through Scenario.read_excel()

[behnam-zakeri]

For some users who aim to import the data by reading that from Excel, there is only one option and that is to import the entire Excel workbook through ixmp.Scenario.read_excel(). This may be unnessary and time-consuming, if a user aims to edit a subset of data in Excel, e.g., only in one parameter, and update that part of data in their database. This type of workflow does not need to import the whole data again each time using read-excel().
It will be useful to add a feature to read_excel() to select a subset of data, either by sheet names, by filtering using the same syntax as ixmp, or combination of both.
For example: ixmp.Scenario.read_excel('filename.xlsx', sheet_name='bar', filters={'node': 'foo'})

[khaeru]

Thanks for noting this @behnam-zakeri!

I removed assignments for myself and @OFR-IIASA because it has not been decided who will work on this.