get rid of npdim

[segasai]

This gets rid of npdim and produces a couple of deprecation warnings.

A couple of decisions I had to make -- I made sure to rename all the npdim= keywords in various classes (other than the top ones like NestedSampler and DynamicNestedSampler) to ndim=. The reason is that I prefer minimize the number of concepts/standard variable names throughout the code.
The consequence of that is that when we release this i don't quite know if restoring checkpoints from previous version will be possible. I don't think it is a big deal, but not everyone may agree.

This is followup from #456

[coveralls]

Pull Request Test Coverage Report for Build 6774895700

• 28 of 34 (82.35%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 91.729%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/dynesty/dynesty.py	9	15	60.0%

Files with Coverage Reduction	New Missed Lines	%
py/dynesty/nestedsamplers.py	1	91.6%

Totals
Change from base Build 6655377848:	-0.1%
Covered Lines:	4159
Relevant Lines:	4534

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💛 - Coveralls

[segasai]

I checked that the checkpoints from previous version will not be restored with this change. The error message

dynesty/py/dynesty/utils.py

Line 2272 in f59d963

warnings.warn(

however is clear enough, so I don't consider that an issue.
I'll merge this soon.