Add the Python implementation of the histogram plot type

[TatianaBurek]

Describe the New Feature

*Implement needed classes and add the histogram plots - Rhist, Phist, Relp properties to METviewer.
phist.R_tmpl (Probability Histogram or Histograms of probability integral transform)
relp.R_tmpl (Relative Histogram or Histograms of relative position)
rhist.R_tmpl (Rank Histogram or Histograms of ensemble rank)

Corresponding METcalcpy issue:
dtcenter/METplotpy#141

Acceptance Testing

The histogram plots should be similar to the Rscript plots and include all the functionalities except of those listed in
https://github.com/dtcenter/METplotpy/wiki

Time Estimate

Estimate the amount of work required here.
Issues should represent approximately 1 week of work.

Sub-Issues

Consider breaking the new feature down into sub-issues.

• Probability Histogram
• Relative Histogram
• Rank Histogram

Relevant Deadlines

List relevant project deadlines here or state NONE.

Funding Source

Define the source of funding and account keys here or state NONE.

Define the Metadata

Assignee

• Select engineer(s) or no engineer required
• Select scientist(s) or no scientist required

Labels

• Select component(s)
• Select priority
• Select requestor(s)

Projects and Milestone

• Select Repository and/or Organization level Project(s) or add alert: NEED PROJECT ASSIGNMENT label
• Select Milestone as the next official version or Future Versions

Define Related Issue(s)

Consider the impact to the other METplus components.

• METplus, MET, METdatadb, METviewer, METexpress, METcalcpy, METplotpy

New Feature Checklist

See the METplus Workflow for details.

• Complete the issue definition above, including the Time Estimate and Funding source.
• Fork this repository or create a branch of develop.
  Branch name: feature_<Issue Number>_<Description>
• Complete the development and test your changes.
• Add/update log messages for easier debugging.
• Add/update unit tests.
• Add/update documentation.
• Push local changes to GitHub.
• Submit a pull request to merge into develop.
  Pull request: feature <Issue Number> <Description>
• Define the pull request metadata, as permissions allow.
  Select: Reviewer(s) and Linked issues
  Select: Repository level development cycle Project for the next official release
  Select: Milestone as the next official version
• Iterate until the reviewer(s) accept and merge your changes.
• Delete your fork or branch.
• Close this issue.