Change detection of ETA in read_nm_tables index_table function to rem…

[MikeKSmith]

…ove digits and search from start of column name. Allows detection of variables like ETA_CL and ETA_V as type "eta". Closes #112

Question: Is it OK to assume that the "ETA" or "ET" comes at the start of the variable name?

[codecov]

Codecov Report

Merging #113 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #113   +/-   ##
=======================================
  Coverage   94.34%   94.34%           
=======================================
  Files          40       40           
  Lines        3057     3057           
=======================================
  Hits         2884     2884           
  Misses        173      173

Impacted Files	Coverage Δ
R/read_nm_tables.R	92.89% <100%> (ø)	⬆️

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[billdenney]

After applying this, my eta_distrib() fails when my ETAs are named "ETA(CL)" and similar:

eta_distrib(xpdb=xpdb)
Error in ETA(CL) : could not find function "ETA"

In my NONMEM model, I'm using:

$ABBR REPLACE ETA(CL)=ETA(2)

This is probably the same issue as #116.