add findcols

[bkamins]

Fixes #3235

If we decide to add it in the next PR I would add #2508

[bkamins]

@nalimilan - the rationale behind adding it is to make it more obvious for users how to select columns by condition by their contents. Currently users need to use e.g. comprehension or findall on eachcol and newcomers do not find it obvious.

[nalimilan]

Sorry for the delay. I'm not totally convinced that this is needed. Is it really so important to save a few characters?

Also, the existing *cols functions have equivalents without the "cols" suffix which operate on rows, and this isn't the case here. Do we want to define findall to operate on rows (and in that case maybe have findallcols instead of findcols)?

[bkamins]

I had similar thoughts. The major question is if we want it (naming decision can come next).
Also, related is if we want filtercols function as discussed in #2508.

The options are:

1. do not add anything;
2. add only column filtering (which extends filter following the rules you described with filtercols naming).
3. add column filtering and a helper findcols function (possibly with a different name).

All these functions are convenience, so I would be OK not to add anything, but I wanted to discuss this, as I see that newcomers might find it easier if such functions were available.