MathML elements svg class for styling and filters

[biojet1]

Added to config .svg_classes .svg_def .svg_style for easy svg styling and filters.
see test/classes.ipynb for example to generate this svg

import ziamath as zm
zm.config.svg2 = False
zm.config.math.fontsize = 50
zm.config.svg_classes = True
zm.config.svg_style = r'''
.mrow { fill:blue }
.mn { fill:magenta ;   }
.mo { fill:yellow ; stroke:grey;   stroke-width:1px;}
.mi { fill:url(#grad1) ; stroke:darkslategrey;   stroke-width:1px;}
.background { fill:violet ; stroke:none}
.hline {fill:green}
'''
zm.config.svg_defs = r'''
    <linearGradient id="grad1" x1="0%" x2="100%" y1="0%" y2="100%">
      <stop offset="0%" stop-color="yellow" />
      <stop offset="100%" stop-color="red" />
    </linearGradient>
'''
mml = '''
<math display="block">
<mrow>
  <mrow>
    <mo>(</mo>
    <mfrac linethickness="1px">
      <mi>p</mi>
      <mn>2</mn>
    </mfrac>
    <mo>)</mo>
  </mrow>
  <msup>
    <mi>x</mi>
    <mn>2</mn>
  </msup>
  <msup>
    <mi>y</mi>
    <mrow>
      <mi mathcolor="DarkSlateGray">p</mi>
      <mo>-</mo>
      <mn>2</mn>
    </mrow>
  </msup>
  <mo>-</mo>
  <mfrac>
    <mn>1</mn>
    <mrow>
      <mn>1</mn>
      <mo>-</mo>
      <mi>x</mi>
    </mrow>
  </mfrac>
  <mfrac>
    <mn>1</mn>
    <mrow>
      <mn>1</mn>
      <mo>-</mo>
      <msup>
        <mi mathbackground="">x</mi>
        <mn mathbackground="orange">2</mn>
      </msup>
    </mrow>
  </mfrac>
</mrow>
</math>
'''
zm.Math(mml)