cemekl, cevmkl: plasma emission, multi-temperature using mekal

A multi-temperature plasma emission model. Emission measures follow a
power-law in temperature (
).The `switch` parameter
determines whether spectrum is calculated by running the mekal code,
by interpolating on a pre-calculated mekal table, or using the apec
model. The final option is now preferred.

For the cemekl version, the abundance ratios are set by the abund command. The cevmkl variant allows the user to define the abundances.

For cemekl the parameters are:

par1 | , index for power-law emissivity function |

par2 | , maximum temperature |

par3 | (cm |

par4 | Abundance relative to Solar |

par5 | z, redshift |

par6 | Switch (0 = calculate, 1 = interpolate, 2 = interpolate using APEC model). |

norm | Normalization |

For the cevmkl variant the parameters are:

par1 | , index for power-law emissivity function |

par2 | , maximum temperature |

par3 | (cm |

par4-17 | Abundances of He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, Ni wrt Solar (defined by the abund command) |

par18 | z, redshift |

par19 | Switch (0 = calculate, 1 = interpolate, 2 = interpolate using APEC model). |

norm | Normalization |