hist_nhtfrq(N)= overrides later hist_nhtfrq=

### Brief summary of bug

(This presumably applies to the other hist_ variables, but I first noticed it with this one.) I have a user_nl_clm file like this:
```
hist_nhtfrq = -24,-8
hist_nhtfrq(3) = -24
hist_nhtfrq = 0,-240,0
```
And it produces:
- h0 file: Monthly, as expected
- h1 file: Every 10 days, as expected
- h2 file: Daily for some reason

If I do
```
hist_nhtfrq = -24,-8,-24
hist_nhtfrq = 0,-240,0
```
then h2 is correctly monthly.

### General bug information

**CTSM version you are using:** ctsm5.2.018

**Does this bug cause significantly incorrect results in the model's science?** It could! I can imagine a scenario where someone has a YEAR-01-01 file they thought was an annual mean for YEAR-1 but was actually the daily mean for YEAR-01-01.

**Configurations affected:** All

### Details of bug

Resolving this would involve dealing with Perl 😱. Maybe would be better to migrate to a more modern namelist parser that a different component is using?

At the very least, we should look through our tests and make sure none of them are being affected by this.