Add support for accumulated fields and fill values; clean-up

[peverwhee]

The main purpose of this PR is to add logic to accumulate fields (average, min, max, standard deviation). Side effects of these changes:

1. Logic relies on whether there are fill values, so I added fill_values to the hist_field_info_t object.
2. All accumulation needs to be done with REAL64 or the numbers are off, so took the opportunity to clean up the hist_buffer interfaces to only have real64 buffers (conversion to real32 happens before writing to a file)
3. I added an interface in hist_api.F90 so that the host model doesn't need to know about buffers.

Modified files:

M    src/hist_api.F90
- add new hist_field_norm_value interfaces (called by host). These, in turn, call hist_buffer_norm_value
- remove unnecessary interfaces - all buffers are now real64, so the only thing that varies is the dimension

M   src/hist_buffer.F90
- remove unnecessary interfaces - all buffers are now real64
- add accumulation logic for min, max, avg, var (standard deviation)
  - includes addition of new var_buffer for standard deviation calculation
  - also includes check_fill_value that confirms that the fill values are consistently applied across a whole column (all vertical layers); errors otherwise
- change num_samples to be a 1d array - we only need to keep track of samples once per column since fill values are ensured to be consistently applied

M    src/hist_field.F90
- add fillvalue to object; add a getter for fill value

[nusbaume]

Thanks for implementing the new history capabilities @peverwhee! I have some change requests, a few of which might take extra effort and thus could be pushed off as future work. Also like always if you want to discuss any of my requests then just let me know!

[nusbaume]

Thanks for addressing my comments @peverwhee! I have one last request for adding an additional error check, but otherwise everything looks good to me. Thanks again!