Support SBML <notes> in Antimony

[mastevb]

Currently, Antimony does not support the tag in SBML models.
For example,

<parameter constant="false" id="CL_rest" metaid="_274131" sboTerm="SBO:0000035" units="l_per_h">
    <notes>
        <p xmlns="http://www.w3.org/1999/xhtml">
        L-Dopa clearance via unspecified elimination pathways</p>
    </notes>
</parameter>

The textual description of the parameter will be lost when converting the model to Antimony.
We would like to support the tag to preserve important information in the model.

We have a few design proposals to consider.

Design Proposal 1

Embed the SBML notes as Antimony comments.
For example, a single-line comment would look like this:

// something about k1
k1 = 0.1

And a multi-line comment would look like this:

/*
something about k1
something else  about k1
*/
k1 = 0.1

Design Proposal 2

Create a new "notes" keyword in Antimony.
A single-line comment would look like this:

k1 = 0.1
k1 notes "something about k1"

Multi-line comments might be a little bit tricky to handle.
One way to handle multi-line comments could be:

k1 notes begin:
    abc
    sdf
    dsdc
notes end

where we use the "notes begin" and "notes end" keyword to signify the start and end of notes.

Design Proposal 3

Create a new operator in Antimony for notes to distinguish regular comments and SBML comments.
A single-line comment would look like this:

k1 = 0.1
k1 //! "something about k1"

We could use the ''' operator for multi-line comments. Example:

k1 '''
    abc
    sdf
    dsdc
'''

Another operator to consider is { and }, example:

model { this is my model
    and my model alone
}