Allow one to set a minimum dimension when truncating. #181
Description
Currently if one employs a truncation strategy that truncates below a tolerance, then it is possible to end up with matrices of dimension 0, which can be inconvenient. It would be useful to be able to set a minimum truncation dimension in truncation strategies to prevent this.
As discussed with @lkdvos, it might be possible to achieve this by allowing for "union" truncation strategies, i.e.
# psuedocode
# assuming reverse ordered SVD spectrum
for (pos, val) in enumerate(spectrum)
truncate = (abs(val) > cutoff && pos >= mindim) || pos > maxdim
end