pandas1.0-ready+ deal w lack of units in char matrix + other bugs
pandas1.0-ready+ deal w lack of units in char matrix + other bugs

- Made ecospold2matrix Pandas 1.0+ ready:
   - Abandon "SparseDataFrame", and use the new convenient SparseDtype approach
   - Replace all df.ix[] and df.reindex_axis() instances

- Starting with ecoinvent 3.4, the characterisation matrix provided by
ecoinvent does not include units, and unit matching therefore caused a
failure. Fixed.

- fix multiple bugs in system export to file

- By default now, replace NaN by zero, and use float64. Can be changed
as parameters.

 On branch dev
 Changes to be committed:
	modified:   ecospold2matrix.py

majeau-bettez/ecospold2matrix