Attempt to port some of the fixes in #50 to ipa_bases.csv

[net]

#50 applied fixes directly to ipa_all.csv without updating ipa_bases.csv. This attempts to port some of the fixes to ipa_bases.csv:

• Updated strident features
• Updated back features
• Removed duplicate vowels (leaving in place the 0delrel versions)

It's unfortunately still not in a place where we can run generate_ipa_all.py and get back the same ipa_all.csv as on master.