Add NMODL Language

[matz-e]

This is the NEURON extension to MODL, a model description language used
within the neuroscientific community. See also:

https://www.neuron.yale.edu/neuron/static/py_doc/modelspec/programmatic/mechanisms/nmodl2.html

Description

I am involved in the neuroscientific community around the NEURON simulator. It has its
own DSL used to describe neuron processes. We have noticed that a lot of the code files
are misclassified as AMPL, and would like to fix that.

The linked search matches 12.7k files, which I think constitutes a strong case to
recognize the language within Github. I have included two samples from the main NEURON
repository showcasing the syntax.

The extension is shared by kernel modules and AMPL. I have looked into providing more
samples for the latter, but I am not sure which licenses these examples would be.
Comparing the two languages, they seem very different. Do you have any advise?

I will contact the author(s) of existing syntax highlighting to see if they can provide
their syntax highlighting under a compatible license.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.mod+NEURON
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/neuronsimulator/nrn/blob/master/docs/nmodl/mod/fornetcon.mod
      • https://github.com/neuronsimulator/nrn/blob/master/docs/nmodl/mod/k3st.mod
    • Sample license(s): BSD
  • I have included a syntax highlighting grammar: [URL to grammar repo]
  • I have added a color
    • Hex value: #00356B
    • Rationale: Color used by Yale, the primary institution at which NEURON was developed.
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

Comparing the two languages, they seem very different. Do you have any advise?

Yup, add a regex to the list of current heuristics for the .mod extension:

linguist/lib/linguist/heuristics.yml

Lines 468 to 474 in 4ac734c

	- extensions: ['.mod']
	rules:
	- language: XML
	pattern: '<!ENTITY '
	- language: Modula-2
	pattern: '^\s*(?i:MODULE|END) [\w\.]+;'
	- language: [Linux Kernel Module, AMPL]

[matz-e]

Comparing the two languages, they seem very different. Do you have any advise?

Yup, add a regex to the list of current heuristics for the .mod extension:

Thanks! I added some keywords that would seem fairly safe. Sorry, did not realize it would be relatively simple. Tested the changes on a local repo, too.

[lildude]

Can you please update the test for this heuristic too.

[matz-e]

Can you please update the test for this heuristic too.

OK, added a line there, bundle exec rake test and the crosstest seem to come back clean

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.

[matz-e]

OK, no problem. Thank you for the help & guidance!