Avoid env namespace mixup

[grantmcdermott]

Fixes #8

Passes devtools::check() on my machine, but please double check on your own.

Two asides:

• Why load the datasets (df_hom and df_het) from did2s in the examples and README? Aren't they bundled with this package too?
• I think you can cut down a number of dependencies (and avoid some of the NSE / namespace clashes as we've seen here), by going fully native on the internal data.table code. LMK if that's something you want me to take a stab at, although I'm quite time constrained at present.

[kylebutts]

@grantmcdermott Removed dependencies with https://github.com/kylebutts/didimputation/commit/0a327749a515292574a7bf54d03fa816da73ca75

[grantmcdermott]

Nice! Thanks for the HU.

i'm pretty time constrained ATM but planning (hoping?) to add some updates to the DID site in a bit.